move the example email templates into the synapse package so that they can be
used as package data, which should mean that all of the packaging mechanisms
(pip, docker, debian, arch, etc) should now come with the example templates.

In order to grandfather in people who relied on the templates being in the old
place, check for that situation and fall back to using the defaults if the
templates directory does not exist.

E2E backups

py3: python3.7 hotfixes

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.

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

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