rbd: retry watch re-registration periodically

Revamp watch code to support retrying watch re-registration:

- add rbd_dev->watch_state for more robust errcb handling
- store watch cookie separately to avoid dereferencing watch_handle
  which is set to NULL on unwatch
- move re-register code into a delayed work and retry re-registration
  every second, unless the client is blacklisted

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Tested-by: Mike Christie <mchristi@redhat.com>