osd/SnapMapper::update_snaps() to handle a missing OBJ_ record

osd/SnapMapper::update_snaps() to handle a missing OBJ_ record

by falling back to add_oid() instead of silently creating an
inconsistent state (OBJ_ without matching SNA_ entries). This
was observed on replicas that had recently recovered objects:
the snap mapper entries created during recovery were not visible
to a subsequent snap-trim repop's update_snaps() call, leaving
the clone with no snap mapper entries. Scrub would then detect
and report the inconsistency as an error.

Promote snap mapper remove_oid/clear_snaps logging to dout(10)
and add apply_op_stats tracing to aid diagnosis of any remaining
stat or snap mapper drift.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>