Stop overwriting server keys with other keys

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.

Fix a bug where we would discard a key result which the origin server is no
longer returning. Fixes #5305.

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>