Limit displaynames and avatar URLs

These end up in join events everywhere, so let's limit them.

Fixes #5079

Rewrite get_server_verify_keys, again.

Allow configuring a range for the account validity startup job

Remove some pointless exception handling

use attr.s for VerifyKeyRequest

Fix ignored filter field in `/messages` endpoint

Speed up room stats background update

We have to do this by re-inserting a background update and recreating
tables, as the tables only get created during a background update and
will later be deleted.

We also make sure that we remove any entries that should have been
removed but weren't due to a race that has been fixed in a previous
commit.

The verify_request deferred already returns a suitable SynapseError, so I don't
really know what we expect to achieve by doing more wrapping, other than log
spam.

Fixes #4278.

Attempt to simplify the logic in get_server_verify_keys by splitting it into
two methods.

because namedtuple is awful

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Due to #5269 we may have extremities in our DB that we shouldn't have,
so lets add a cleanup task such to remove those.

When we receive a soft failed event we, correctly, *do not* update the
forward extremity table with the event. However, if we later receive an
event that references the soft failed event we then need to remove the
soft failed events prev events from the forward extremities table,
otherwise we just build up forward extremities.

Fixes #5269