MB-11775: added more logging around stopping rebalance

Change-Id: I1333cbff6a01a27c4d4719e2411918ba79033a17
Reviewed-on: http://review.couchbase.org/39692
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>