Unix socket support for `federation` and `client` Listeners has existed now for a little while(since [1.81.0](https://github.com/matrix-org/synapse/pull/15353)), but there was one last hold out before it could be complete: HTTP Replication communication. This should finish it up. The Listeners would have always worked, but would have had no way to be talked to/at.
---------
Co-authored-by: Eric Eastwood <madlittlemods@gmail.com>
Co-authored-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>
Co-authored-by: Eric Eastwood <erice@element.io>
All the information needed is already in the `instance_map`, so
use that instead of passing the hostname / IP & port manually
for each replication request.
This consolidates logic for future improvements of using e.g.
UNIX sockets for workers.
Separate out a HTTP client for replication in preparation for
also supporting using UNIX sockets. The major difference from
the base class is that this does not use treq to handle HTTP
requests.
Jason Little