Broker should stop accepting work when trying to shut down transports.

During a broker shutdown (as part of failover after losing the DB lease) the broker was blocked for some unknown reason shutting down the transports.To mitigate against shutdown delays/issues, it would be good to introduce a concept of rejecting new work during broker shutdown. If the original master broker is delayed shutting down and still processing work there is potential for two active brokers on the same store( two "masters").Discussed this with gtully; perhaps this could be achieved with a broker filter that could be activated as the first step of Broker shutdown. If delays are experienced in subsequent shutdown steps at least no new work would be processed.