rbd: introduce image request state machine

Make it possible to schedule image requests on a workqueue.  This fixes
parent chain recursion added in the previous commit and lays the ground
for exclusive lock wait/wake improvements.

The "wait for pending subrequests and report first nonzero result" code
is generalized to be used by object request state machine.

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