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.