rbd: new request handling code

The notable changes are:

- instead of explicitly stat'ing the object to see if it exists before
  issuing the write, send the write optimistically along with the stat
  in a single OSD request
- zero copyup optimization
- all object requests are associated with an image request and have
  a valid ->img_request pointer; there are no standalone (!IMG_DATA)
  object requests anymore
- code is structured as a state machine (vs a bunch of callbacks with
  implicit state)
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>