just some docstrings to clarify the behaviour here

Add metric to count lazy member sync requests

Log looping call exceptions

Optimisation for filter_events_for_server

We're better off hashing just the event_id than the whole ((type, state_key),
event_id) tuple - so use a dict instead of a set.

Also, iteritems > items.

If a looping call function errors, then it kills the loop entirely.
Currently it throws away the exception logs, so we should make it
actually log them.

Fixes #3929

Fix bug in forward_extremity update logic

Pin to prometheus_client<0.4 to avoid renaming all of our metrics

Docker build all tags starting vX.Y.Z, including release candidates

Fix handling of rejected threepid invites

Fix exception in background metrics collection

Fix exception handling in fetching remote profiles

Note the regex must match the complete string anyway,
so the leading ^ was useless.

We attempted to iterate through a list on a separate thread without
doing the necessary copying.

An event does not stop being a forward_extremity just because an outlier or
rejected event refers to it.