Since we're not doing refresh tokens any more, we should start killing off the
dead code paths. /tokenrefresh itself is a bit of a thornier subject, since
there might be apps out there using it, but we can at least not generate
refresh tokens on new logins.

Remove redundant list of known caveat prefixes

Correctly handle 500's and 429 on federation

Also add some comments.

More efficient notif count queries

Split out federation transaction sending to a worker

Ignore AS users when fetching push rules

By ignoring AS users early on when fetching push rules for a room we can
avoid needlessly hitting the DB and filling up the caches.

Return early on /sync code paths if a '*' filter is used

This is currently very conservative in that it only does this if there is no
`since` token. This limits the risk to clients likely to be doing one-off
syncs (like bridges), but does mean that normal human clients won't benefit
from the time savings here. If the savings are large enough, I would consider
generalising this to just check the filter.

Implement "event_fields" in filters

Merge branch 'dbkr/work_around_devicename_bug' of github.com:matrix-org/synapse into release-v0.18.4

Also make it an inclusive not exclusive filter, as the spec demands.