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.

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

Fix error handling for missing auth_event

Fix error when logging incomplete requests

If a connection is lost before a request is read from Request, Twisted
sets `method` (and `uri`) attributes to dummy values. These dummy values
have incorrect types (i.e. they're not bytes), and so things like
`__repr__` would raise an exception.

To fix this we had a helper method to return the method with a
consistent type.

Avoid reraise, to improve stacktraces

Fix lazy loaded sync with rejected state events