MB-100: logging bucket name in StatsWaitTask logs

Change-Id: If0436cfbe8c7b33a738be20d0b488e592efe7642
Reviewed-on: http://review.couchbase.org/25258
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>