s4-repl: don't do double replication

s4-repl: don't do double replication

When we replicate from a remote DC, we need to note the new uSN that
the local changes have resulted in, and modify the uSN that the notify
task uses to determine if it should send a ReplicaSync message back to
the remote DC. Otherwise we end up always triggering a ReplicaSync
every time we replicate from another DC