ReplicatedPG: do not create whiteout clones

First, make_writeable treats whiteout heads like snapdir for
cloning purposes.  Second, to ensure that we send the correct
deletes on flush to the backing pool, we instead use oi.snaps
on any clone we are flushing to infer the snaps during which
head did not exist and send a delete as appropriate prior to
the copy_from.

Normally, we'd have a problem if the delete and the copy_from
completed, but an interval change intervened before the dirty
flag was cleared since we'd end up re-deleting the object.
To avoid that, we use the CEPH_OSD_FLAG_ORDERSNAP flag.

Additionally, we will use the correct snap_seq on the delete
or flush as appropriate to ensure that the previous clone
gets created with the same clone id as in the cache pool.

Fixes: #7942
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: if we get ENOENT on clone, remove clone from snapset

Fixes: #7916
Signed-off-by: Samuel Just <sam.just@inktank.com>