Adds a new method, check_3pid_auth, which gives password providers
the chance to allow authentication with third-party identifiers such
as email or msisdn.

Redo of https://github.com/matrix-org/synapse/pull/4840

`__str__` depended on `self.addr`, which was absent from
ClientReplicationStreamProtocol, so attempting to call str on such an object
would raise an exception.

We can calculate the peer addr from the transport, so there is no need for addr
anyway.

Make sure that they are sent correctly over the replication stream.

Fixes: #4898

I don't have a database with the same name as my user, so leaving the database
name unset fails.

While we're at it, clear out some unused stuff in the test setup.

Fix yaml warnings by using safe_load

As per #3622, we remove trailing slashes from outbound federation requests. However, to ensure that we remain backwards compatible with previous versions of Synapse, if we receive a HTTP 400 with `M_UNRECOGNIZED`, then we are likely talking to an older version of Synapse in which case we retry with a trailing slash appended to the request path.

Deny peeking into rooms that have been blocked

Co-Authored-By: erikjohnston <erikj@jki.re>

This one should close #4841. Many thanks to @dev4223 for bringing it up and finding a solution.

Signed-off-by: Colin White

Fixup shutdown room API

Add option to disable search room lists

Rather than stubbing out the access_log, make it actually log the requests,
which makes it a lot more obvious what is going on during tests.