rbd: implement watch/unwatch with new code

Implement a new function to set up or tear down a watch event
for an mapped rbd image header using the new request code.

Create a new object request type "nodata" to handle this.  And
define rbd_osd_trivial_callback() which simply marks a request done.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>