Add a remote config to exclude replicating desired refs

Allow admins to exclude specific refs from replication when they are
not needed on replicas. For instance, refs created by certain plugins
can now be skipped. This config helps improve replication performance
and stability, especially on busy servers that create a large number
of such unwanted refs.

Change-Id: I42366148cfb1f9da35904ad8e07d3fc103966a53