rbd-mirror: make image replayer asok commands available when not started

rbd-mirror: make image replayer asok commands available when not started

Initially the asok commands were registered only after the image
replayer start (and unregistered on stop) because their names were
built using remote pool and image names, which became known only after
start.

Now, the asok commands are registered on the image replayer construction
using the temporary name "remote_pool_name/global_image_id". They are
re-registered using "remote_pool_name/remote_image_name" when the
image replayer is started.  Also the commands are not unregistered on
the image replayer stop.

Signed-off-by: Mykola Golub <mgolub@mirantis.com>
(cherry picked from commit 21790484dc66e97e7625b5b2afabb7efe3a92b08)