ReplicatedPG: take clone write lock as well in make_writeable

Otherwise, we might start backfilling the clone before the op is
complete.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: release backfill state on_global_recover not peer recover

There may now be multiple backfill peers.

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