MB-100: change invocation point for rebalancer thread

Change-Id: Ic943e300dbb629285604902e6b6f70fd1f5b17e6
Reviewed-on: http://review.couchbase.org/23882
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>