MB-11775: log all process backtraces on slow rebalance stop

So that we can diagnose MB-11775 better.

Change-Id: I3b845d6fd6d6fa8ddd9b91565176058af25197a6
Reviewed-on: http://review.couchbase.org/39625
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>