MB-9343 allow to stop rebalance if it hanged in some janitor_agent

.. calls

Change-Id: I1ae9b3bca9368a3d8bda53fe9242bc6d1c278222
Reviewed-on: http://review.couchbase.org/34691
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>