Fanout protocol: allow consuming from multiple brokers

The fanout URL is originally designed for producing into multiple brokers at once. But an enhancement to consume from multiple brokers simultaneously would be nice. Currently a 'primary' connection is selected out of all underlying transport connections, and all consumption is attached to the 'primary' connection.

This behaviour was already requested in the community: http://activemq.2283324.n4.nabble.com/Single-consumer-for-multiple-brokers-td2362045.html. But I've been unable to find any follow-ups, hence creating this ticket.