librbd/object_map: diff state machine should track object existence

librbd/object_map: diff state machine should track object existence

The deep-copy snapshot-create state machine initializes the object-map
state to non-existent for all objects. There was an assumption that the
deep-copy object-copy state machine would always update the object map
but that was being skipped for clean objects as an optimization. This
change will support a future commit to run the object-copy state machine
for existing objects.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit b81cd2460de748c71210520f8c819895f257f0c7)

Conflicts:
	src/librbd/api/DiffIterate.cc: trivial resolution due to renames