Improve comments on get_user_by_access_token

Delete devices and pushers on logouts etc

Initial commit; this doesn't work yet - the LIKE filtering seems too aggressive.
It also needs _do_initial_spam to be aware of prepopulating the whole user_directory_search table with all users...
...and it needs a handle_user_signup() or something to be added so that new signups get incrementally added to the table too.

Committing it here as a WIP

Make sure that we delete devices whenever a user is logged out due to any of
the following situations:

 * /logout
 * /logout_all
 * change password
 * deactivate account (by the user or by an admin)
 * invalidate access token from a dynamic module

Fixes #2672.

Non-functional refactoring to move set_password. This means that we'll be able
to properly deactivate devices and access tokens without introducing a
dependency loop.

Non-functional refactoring to move deactivate_account. This means that we'll be
able to properly deactivate devices and access tokens without introducing a
dependency loop.

Whenever an access token is invalidated, we should remove the associated
pushers.

Clear logcontext before starting fed txn queue runner

Fix 500 when joining matrix-dev

matrix-dev has an event (`$/6ANj/9QWQyd71N6DpRQPf+SDUu11+HVMeKSpMzBCwM:zemos.net`)
which has no `hashes` member.

Check for missing `hashes` element in events.

because I have to reverse-engineer this every time.

These processes take a long time compared to the request, so there is lots of
"Entering|Restoring dead context" in the logs. Let's try to shut it up a bit.

Allow guest access to group APIs for reading

Fix error on sqlite 3.7

Avoid retrying forever on IntegrityError

Fix error handling on dns lookup

Tiny code cleanups

Fix some logcontext leaks in replication resource

Fix OPTIONS on preview_url

Improve tracebacks on exceptions

Use failure.Failure to recover our failure, which will give us a useful
stacktrace, unlike the rethrown exception.

pass the right arguments to the errback handler

Fixes "TypeError('eb() takes exactly 2 arguments (1 given)',)"

This is never used; let's remove it to stop confusing things.