rbd: introduce rbd_obj_issue_copyup_ops()

In preparation for deep-flatten feature, split rbd_obj_issue_copyup()
into two functions and add a new write state to make the state machine
slightly more clear.  Make the copyup op optional and start using that
for when the overlap goes to 0.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>