It's still not used, however the parameter is an event ID not a transaction ID.

Fix some looping_call calls which were broken in #3604

It turns out that looping_call does check the deferred returned by its
callback, and (at least in the case of client_ips), we were relying on this,
and I broke it in #3604.

Update run_as_background_process to return the deferred, and make sure we
return it to clock.looping_call.

Implement the lazy_load_members room state filter parameter

Fix a minor documentation typo in on_make_leave

 Correctly handle outliers during persist events

Fix another logcontext leak in _persist_events

Fix occasional 'tuple index out of range' error

Fix updating of cached remote profiles

Wrap a number of things that run in the background

This fixes a bug in _delete_existing_rows_txn which was introduced in #3435
(though it's been on matrix-org-hotfixes for *years*). This code is only called
when there is some sort of conflict the first time we try to persist an event,
so it only happens rarely. Still, the exceptions are annoying.

We need to run the errback in the sentinel context to avoid losing our own
context.

Also: add logging to runInteraction to help identify where "Starting db
connection from sentinel context" warnings are coming from

_update_remote_profile_cache was missing its `defer.inlineCallbacks`, so when
it was called, would just return a generator object, without actually running
any of the method body.

on_notifier_poke no longer runs synchonously, so we have to do a different hack
to make sure that the replication data has been sent. Let's actually listen for
its arrival.

This will reduce the number of "Starting db connection from sentinel context"
warnings, and will help with our metrics.

We incorrectly asserted that all contexts must have a non None state
group without consider outliers. This would usually be fine as the
assertion would never be hit, as there is a shortcut during persistence
if the forward extremities don't change.

However, if the outlier is being persisted with non-outlier events, the
function would be called and the assertion would be hit.

Fixes #3601

Fix client_reader worker being able to handle /context requests

This allows us to handle /context/ requests on the client_reader worker
without having to pull in all the various stream handlers (e.g.
precence, typing, pushers etc). The only thing the token gets used for
is pagination, and that ignores everything but the room portion of the
token.

Use deltas to calculate current state deltas