tools/cephfs_mirror: Store a reference of PeerReplayer object in SyncMechanism
Store a reference of PeerReplayer object in SyncMechanism.
This allows SyncMechansim object to call functions of PeerReplayer.
This is required in multiple places like handling
shutdown/blocklist/cancel where should_backoff() needs to be
called by syncm object while poppig dataq by data sync threads.