MB-100: log master events after rebalance

Change-Id: If987a0f27d5b74e3b064dbffeead5275a97e9cf1
Reviewed-on: http://review.couchbase.org/24005
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>