ReplicatedPG/PG: check snap collections during _scan_list

During _scan_list check the snapcollections corresponding to the
object_info attr on the object.  Report inconsistencies during
scrub_finalize.

--Needs more testing--

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: compare nlinks to snapcolls

nlinks gives us the number of hardlinks to the object.
nlinks should be 1 + snapcolls.size().  This will allow
us to detect links which remain in an erroneous snap
collection.

Signed-off-by: Samuel Just <sam.just@inktank.com>