rbd-mirror: schedule rebalancer to level-load instances

rbd-mirror: schedule rebalancer to level-load instances

Policy implementation takes care of evenly balancing images
across rbd mirror instances. This is done when images are
added to the map and/or instances are added or removed with
the exception of image removal -- removing images does not
reshuffle other (mapped) images which can result in some of
the instances under loaded (in worst case, if one removes
images which all map to a particular instance, that instance
would remain idle until more images are added or a shuffle is
triggered).

We could possibly trigger map shuffle when images are removed,
but that would change the interface between Policy and ImageMap
class (in the form of changes to Policy::remove_images()). Also,
policy (and its implementations) would have to do more work when
the above class method is invoked.

Therefore, an interval based rebalancer is added to ImageMap for
periodic rebalancing of images only if the following conditions
are met:

    - policy has been idle for a configured time duration
    - no scheduled or in-transit operations

Signed-off-by: Venky Shankar <vshankar@redhat.com>