Ignore room state with null bytes in for room stats

It takes at least 20 minutes to work through the long_retries schedule (11
attempts, each with a 60 second timeout, and 60 seconds between each request),
so if the notary server isn't returning within the timeout, we'll just end up
blocking whatever request is happening for 20 minutes.

Ain't nobody got time for that.

Fixes for the key-notary server

... else we're guaranteed to time out.

When handling incoming federation requests, make sure that we have an
up-to-date copy of the signing key.

We do not yet enforce the validity period for event signatures.

This reverts commit 4bd67db1.

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.