ReplicatedPG::start_flush: send delete even if there are no snaps

Even if all snaps for the clone have been removed, we still have to
send the delete to ensure that when the object is recreated the
new snaps aren't included in the wrong clone.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::start_flush: fix clone deletion case

dsnapc.snaps will be non-empty most of the time if there
have been snaps before prev_snapc.  What we really want to
know is whether there are any snaps between oi.snaps.back()
and prev_snapc.

Fixes: 8334
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1834 from ceph/wip-8334

Wip 8334

Reviewed-by: Sage Weil <sage@inktank.com>