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.

Since we don't actually delete the keys, just mark the versions
as deleted in the db rather than actually deleting them, then we
won't reuse versions.

Fixes https://github.com/vector-im/riot-web/issues/7448

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.

Fix bug when invalidating destination retry timings