Allow to filter out refs from the fetch-replication deltas

Other plugins, e.g. the global-refdb, may want to influence the behaviour
of the pull-replication plugin and prevent refs from being replicated.

Introduce the refs filtering interface defined already in
the replication plugin with I99a7cb4fb2 but applied to the refs
deltas to fetch.

Bug: Issue 15637
Change-Id: I19d58638181fa4bab204aba30b2cda73241f7ed0