Miscellaneous fixes to python_dependencies

Improve exception handling for background processes

Add missing consumeErrors to improve exception handling

In general we want defer.gatherResults to consumeErrors, rather than having
exceptions hanging around and getting logged as CRITICAL unhandled errors.

There were a bunch of places where we fire off a process to happen in the
background, but don't have any exception handling on it - instead relying on
the unhandled error being logged when the relevent deferred gets
garbage-collected.

This is unsatisfactory for a number of reasons:
 - logging on garbage collection is best-effort and may happen some time after
   the error, if at all
 - it can be hard to figure out where the error actually happened.
 - it is logged as a scary CRITICAL error which (a) I always forget to grep for
   and (b) it's not really CRITICAL if a background process we don't care about
   fails.

So this is an attempt to add exception handling to everything we fire off into
the background.

Changes in synapse v0.28.0-rc1 (2018-04-26)
===========================================

Bug Fixes:

* Fix quarantine media admin API and search reindex (PR #3130)
* Fix media admin APIs (PR #3134)

Changes in synapse v0.28.0-rc1 (2018-04-24)
===========================================

Minor performance improvement to federation sending and bug fixes.

(Note: This release does not include state resolutions discussed in matrix live)

Features:

* Add metrics for event processing lag (PR #3090)
* Add metrics for ResponseCache (PR #3092)

Changes:

* Synapse on PyPy (PR #2760) Thanks to @Valodim!
* move handling of auto_join_rooms to RegisterHandler (PR #2996) Thanks to @krombel!
* Improve handling of SRV records for federation connections (PR #3016) Thanks to @silkeh!
* Document the behaviour of ResponseCache (PR #3059)
* Preparation for py3 (PR #3061, #3073, #3074, #3075, #3103, #3104, #3106, #3107, #3109, #3110) Thanks to @NotAFile!
* update prometheus dashboard to use new metric names (PR #3069) Thanks to @krombel!
* use python3-compatible prints (PR #3074) Thanks to @NotAFile!
* Send federation events concurrently (PR #3078)
* Limit concurrent event sends for a room (PR #3079)
* Improve R30 stat definition (PR #3086)
* Send events to ASes concurrently (PR #3088)
* Refactor ResponseCache usage (PR #3093)
* Clarify that SRV may not point to a CNAME (PR #3100) Thanks to @silkeh!
* Use str(e) instead of e.message (PR #3103) Thanks to @NotAFile!
* Use six.itervalues in some places (PR #3106) Thanks to @NotAFile!
* Refactor store.have_events (PR #3117)

Bug Fixes:

* Return 401 for invalid access_token on logout (PR #2938) Thanks to @dklug!
* Return a 404 rather than a 500 on rejoining empty rooms (PR #3080)
* fix federation_domain_whitelist (PR #3099)
* Avoid creating events with huge numbers of prev_events (PR #3113)
* Reject events which have lots of prev_events (PR #3118)

* add some doc about wtf this thing does
* pin Twisted to < 18.4
* add explicit dep on six (fixes #3089)

Fix media admin APIs

Fix quarantine media admin API

Rephrase v0.28.0-rc1 summary

fix formatting on line break

This reverts commit 08b29d45.

Reject events which have lots of prev_events

Make tests py3 compatible

Use six.itervalues in some places

add __bool__ alias to __nonzero__ methods

Add some more variables to the unittest config

Replace Queue with six.moves.queue

Use str(e) instead of e.message

Clarify that SRV may not point to a CNAME

Document contrib directory

Refactor ResponseCache usage

Refactor store.have_events

It turns out that most of the time we were calling have_events, we were only
using half of the result. Replace have_events with have_seen_events and
get_rejection_reasons, so that we can see what's going on a bit more clearly.

Avoid creating events with huge numbers of prev_events