tools/cephfs_mirror: Load persisted mirror metrics from omap

tools/cephfs_mirror: Load persisted mirror metrics from omap

Load last_synced_snap metadata from the cephfs_mirror object omap on
PeerReplayer initialization and when a mirrored directory is added.
Live current_syncing_snap metrics are not restored; they are rebuilt
when synchronization starts.

When the daemon restarts after a snapshot was synced on the remote but
metrics were not yet written to omap, loaded metadata may belong to an
older snapshot.  Add reconcile_last_synced_snap() to compare against
the remote snap map, clear stale last-sync fields, and update
last_synced_snap id/name in memory.

Treat snaps_synced, snaps_deleted, and snaps_renamed as per-session
counters.  Do not load them from omap; they start at zero for each
daemon session and are still reported via the admin socket.  Persist
omap metrics unconditionally after reconcile so the mgr picks up the
new instance id and cleared session counters on restart.

Fixes: https://tracker.ceph.com/issues/76686
Signed-off-by: Kotresh HR <khiremat@redhat.com>