[AMQ7, exclusive consumer] add exclusive consumer functionality to AMQ 7

Customer asking for AMQ6 exclusive consumer functionality to be included in AMQ 7. The motivation for this requirement is to have:

• multiple consumers connected to a destination but only one of those consumers should be receiving all the messages.
• the selection of the consumer is decided by the broker
• if that consumer is no longer active on the queue the broker should select another active consumer that is connected to the queue
• want this to work with all protocols.

Customer has already considered and dismissed:

• using "message groups"; This bleeds into the application producer - producer has to set the group name before the message is sent
• using "max-consumers": This involves the application reconnecting the consumer after the initial consumer has disappeared.

Customer would like this "exclusive consumer" behaviour to be dynamic and not require configuration changes to enable this behaviour - that would result in broker downtime.