MB-100: assign an appropr name of the Cluster thread

Change-Id: I75bfc4e5bd34c192fb1adff53f1a5581bebef891
Reviewed-on: http://review.couchbase.org/23910
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>