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.

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