This avoids us from uncessarily hitting the database when there has been
no change for the room

Let's try to include time spent in the DB threads in the per-request/block cpu
usage metrics.

Factor out the resource usage tracking out to a separate object, which can be
passed around and copied independently of the logcontext itself.

Add CPU metrics for _fetch_event_list

add a Measure block on _fetch_event_list, in the hope that we can better
measure CPU usage here.

Run isort on Synapse

Synapse 0.32.1 (2018-07-06)
===========================

Bugfixes
--------

- Add explicit dependency on netaddr ([#3488](https://github.com/matrix-org/synapse/issues/3488))

Add explicit dependency on netaddr

the dependencies file, causing failures on upgrade (and presumably for new
installs).

More server_name validation

... as described at
https://docs.google.com/document/d/1EttUVzjc2DWe2ciw4XPtNpUpIl9lWXGEsy2ewDS7rtw.

We need to do a bit more validation when we get a server name, but don't want
to be re-doing it all over the shop, so factor out a separate
parse_and_validate_server_name, and do the extra validation.

Also, use it to verify the server name in the config file.

Reinstate lost run_on_reactor in unit test

Invalidate cache on correct thread

Fix up auth check

a61738b3 removed a call to run_on_reactor from a unit test, but that call was
doing something useful, in making the function in question asynchronous.

Reinstate the call and add a check that we are testing what we wanted to be
testing.

Make sure that server_names used in auth headers are sane, and reject them with
a sensible error code, before they disappear off into the depths of the system.

don't mix unicode strings with utf8-in-byte-strings