MB-11775: anticipate rebalance stop while verifying replications

Evidence suggests that reason for slow rebalance stop in MB-11775 is
due to stuck verify_replication phase. I'm moving it out to separate
child process and anticipate stop message while waiting for that child
process.

Change-Id: I44efde4638e13b49808c78cf46e4a01cc0215737
Reviewed-on: http://review.couchbase.org/39694
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>