rbd: introduce copyup state machine

Both write and copyup paths will get more complex with object map.
Factor copyup code out into a separate state machine.

While at it, take advantage of obj_req->osd_reqs list and issue empty
and current snapc OSD requests together, one after another.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>