Kill rebalance subprocesses before the replicators.

Because some of them expect replicators to be running and hence might
crash and cause entire janitor_agent to crash.

Seen in MB-11890.

Change-Id: I142cbc685d74197b9209264e1bbe5f0d4554dc3d
Reviewed-on: http://review.couchbase.org/40386
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>