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.
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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.
