Fix some instances of ExpiringCache not expiring cache items

Synapse doesn’t allow for media resources to be played directly from
Chrome. It is a problem for users on other networks (e.g. IRC)
communicating with Matrix users through a gateway. The gateway sends
them the raw URL for the resource when a Matrix user uploads a video
and the video cannot be played directly in Chrome using that URL.

Chrome argues it is not authorized to play the video because of the
Content Security Policy. Chrome checks for the "media-src" policy which
is missing, and defauts to the "default-src" policy which is "none".

As Synapse already sends "object-src: 'self'" I thought it wouldn’t be
a problem to add "media-src: 'self'" to the CSP to fix this problem.

Fix spurious exceptions when client closes conncetion

We require attrs 16.0.0

Ref: https://github.com/matrix-org/synapse/issues/3945

Given we have disabled lazy loading for incr syncs in #3840, we can make self-LL more efficient by only doing it on initial sync.  Also adds a bounds check for if/when we change our mind, so that we don't try to include LL members on sync responses with no timeline.

Internal Changes
----------------

- Fix incompatibility with older Twisted version in tests. Thanks
  @OlegGirko!
([\#3940](https://github.com/matrix-org/synapse/issues/3940))

Older Twisted (18.4.0) returns TimeoutError instead of
ConnectingCancelledError when connection times out.
This change allows tests to be compatible with this behaviour.

Signed-off-by: Oleg Girko <ol@infoserver.lv>

Features
--------

- Python 3.5 and 3.6 support is now in beta.
([\#3576](https://github.com/matrix-org/synapse/issues/3576))
- Implement `event_format` filter param in `/sync`
([\#3790](https://github.com/matrix-org/synapse/issues/3790))
- Add synapse_admin_mau:registered_reserved_users metric to expose
number of real reaserved users
([\#3846](https://github.com/matrix-org/synapse/issues/3846))

Bugfixes
--------

- Remove connection ID for replication prometheus metrics, as it creates
a large number of new series.
([\#3788](https://github.com/matrix-org/synapse/issues/3788))
- guest users should not be part of mau total
([\#3800](https://github.com/matrix-org/synapse/issues/3800))
- Bump dependency on pyopenssl 16.x, to avoid incompatibility with
recent Twisted.
([\#3804](https://github.com/matrix-org/synapse/issues/3804))
- Fix existing room tags not coming down sync when joining a room
([\#3810](https://github.com/matrix-org/synapse/issues/3810))
- Fix jwt import check
([\#3824](https://github.com/matrix-org/synapse/issues/3824))
- fix VOIP crashes under Python 3 (#3821)
([\#3835](https://github.com/matrix-org/synapse/issues/3835))
- Fix manhole so that it works with latest openssh clients
([\#3841](https://github.com/matrix-org/synapse/issues/3841))
- Fix outbound requests occasionally wedging, which can result in
federation breaking between servers.
([\#3845](https://github.com/matrix-org/synapse/issues/3845))
- Show heroes if room name/canonical alias has been deleted
([\#3851](https://github.com/matrix-org/synapse/issues/3851))
- Fix handling of redacted events from federation
([\#3859](https://github.com/matrix-org/synapse/issues/3859))
-  ([\#3874](https://github.com/matrix-org/synapse/issues/3874))
- Mitigate outbound federation randomly becoming wedged
([\#3875](https://github.com/matrix-org/synapse/issues/3875))

Internal Changes
----------------

- CircleCI tests now run on the potential merge of a PR.
([\#3704](https://github.com/matrix-org/synapse/issues/3704))
- http/ is now ported to Python 3.
([\#3771](https://github.com/matrix-org/synapse/issues/3771))
- Improve human readable error messages for threepid
registration/account update
([\#3789](https://github.com/matrix-org/synapse/issues/3789))
- Make /sync slightly faster by avoiding needless copies
([\#3795](https://github.com/matrix-org/synapse/issues/3795))
- handlers/ is now ported to Python 3.
([\#3803](https://github.com/matrix-org/synapse/issues/3803))
- Limit the number of PDUs/EDUs per federation transaction
([\#3805](https://github.com/matrix-org/synapse/issues/3805))
- Only start postgres instance for postgres tests on Travis CI
([\#3806](https://github.com/matrix-org/synapse/issues/3806))
- tests/ is now ported to Python 3.
([\#3808](https://github.com/matrix-org/synapse/issues/3808))
- crypto/ is now ported to Python 3.
([\#3822](https://github.com/matrix-org/synapse/issues/3822))
- rest/ is now ported to Python 3.
([\#3823](https://github.com/matrix-org/synapse/issues/3823))
- add some logging for the keyring queue
([\#3826](https://github.com/matrix-org/synapse/issues/3826))
- speed up lazy loading by 2-3x
([\#3827](https://github.com/matrix-org/synapse/issues/3827))
- Improved Dockerfile to remove build requirements after building
reducing the image size.
([\#3834](https://github.com/matrix-org/synapse/issues/3834))
- Disable lazy loading for incremental syncs for now
([\#3840](https://github.com/matrix-org/synapse/issues/3840))
- federation/ is now ported to Python 3.
([\#3847](https://github.com/matrix-org/synapse/issues/3847))
- Log when we retry outbound requests
([\#3853](https://github.com/matrix-org/synapse/issues/3853))
- Removed some excess logging messages.
([\#3855](https://github.com/matrix-org/synapse/issues/3855))
- Speed up purge history for rooms that have been previously purged
([\#3856](https://github.com/matrix-org/synapse/issues/3856))
- Refactor some HTTP timeout code.
([\#3857](https://github.com/matrix-org/synapse/issues/3857))
- Fix running merged builds on CircleCI
([\#3858](https://github.com/matrix-org/synapse/issues/3858))
- Fix typo in replication stream exception.
([\#3860](https://github.com/matrix-org/synapse/issues/3860))
- Add in flight real time metrics for Measure blocks
([\#3871](https://github.com/matrix-org/synapse/issues/3871))
- Disable buffering and automatic retrying in treq requests to prevent
timeouts. ([\#3872](https://github.com/matrix-org/synapse/issues/3872))
- mention jemalloc in the README
([\#3877](https://github.com/matrix-org/synapse/issues/3877))
- Remove unmaintained "nuke-room-from-db.sh" script
([\#3888](https://github.com/matrix-org/synapse/issues/3888))

ExpiringCache required that `start()` be called before it would actually
start expiring entries. A number of places didn't do that.

This PR removes `start` from ExpiringCache, and automatically starts
backround reaping process on creation instead.

Handle exceptions thrown by background tasks

Should fix https://github.com/vector-im/riot-web/issues/7209

Fixes #3921

If a HTTP handler throws an exception while processing a request we
automatically write a JSON error response. If the handler had already
started writing a response twisted throws an exception.

We should check for this case and simple abort the connection if there
was an error after the response had started being written.

Neilj/fix room invite mail links

Remove get_destination_retry_timings cache

Let's try to rationalise the logging that happens when we are processing an
incoming transaction, to make it easier to figure out what is going wrong when
they take ages. In particular:

- make everything start with a [room_id event_id] prefix
- make sure we log a warning when catching exceptions rather than just turning
  them into other, more cryptic, exceptions.

Currently we rely on the master to invalidate this cache promptly.
However, after having moved most federation endpoints off of master this
no longer happens, causing outbound fedeariont to get blackholed.

Fixes #3798

Update to use new timeout function everywhere.