osd/PrimaryLogPG: find_object_context: trust SnapSet's clone_snaps

osd/PrimaryLogPG: find_object_context: trust SnapSet's clone_snaps

Instead of checking the OSDMap pg_pool_t whether a snap exists, instead

1- Look at the clone_snaps more carefully.  If the snap didn't exist when
the clone was last touched (created or partially-trimmed) then it still
doesn't exist now (snaps aren't resurrected).

2- Check in the OSDMap's removed snaps queue.  This will catch anything
that is still being removed but hasn't been reflected by the clone_snaps
yet.

Signed-off-by: Sage Weil <sage@redhat.com>