Move to using TCP replication

Deduplicate new deviceinbox rows for replication

Advance replication streams even if nothing is listening

Otherwise the streams don't advance and steadily fall behind, so when a
worker does connect either a) they'll be streamed lots of old updates or
b) the connection will fail as the streams are too far behind.

Replace HTTP replication with TCP replication (Server side part)

Cull spurious PreserveLoggingContexts

Accept join events from all servers

Make sure that we accept join events from any server, rather than just the
origin server, to make the federation join dance work correctly.

(Fixes #1893).

In `MessageHandler`, remove `yield` on call to `Notifier.on_new_room_event`:
it doesn't return anything anyway.

Remove `PreserveLoggingContext` around calls to `Notifier.on_new_room_event`;
there is no problem if the logcontext is set when calling it.

In `on_new_room_event`, remove `PreserveLoggingContext` - we can call its
subroutines with the logcontext set.

In `Notifier._on_new_room_event`, `preserve_fn` around its subroutines which
return deferreds, so that it is safe to call it with an active logcontext.

`preserve_fn` is no longer used as a decorator anywhere, so we can safely fix a
fixme therein.

As the TCP replication uses a slightly different API and streams than
the HTTP replication.

This breaks HTTP replication.

Remove `@preserve_fn` decorators on `on_new_room_event`,
`_notify_pending_new_room_events`, `_on_new_room_event`, `on_new_event`, and
`on_new_replication_data` - none of these functions return a deferred, and the
decorator does nothing unless the wrapped function returns a deferred, so the
decorator was a no-op.

This timestamp is used to indicate when the user last sync'd

add user friendly report of assertion error in synctl.py

Add missing package to CentOS section

add setting (on by default) to support TURN for guests

enable guest access for the 3pl/3pid APIs

Don't assume postgres tables are in the public schema during db port

Clarify doc for SQLite to PostgreSQL port

bring nuke-room script to current schema