support better load balancing of destinations over brokers

[jstrachan]

when we allocate a destination to a broker it would be nice to use a more fair load balancing technique. e.g. to use some kind of calculation of load (message size on existing queues/topics maybe?) to find the least loaded broker to use? Worst case we should find the one with the least destination count; but then load on a destination will vary wildly so using a better metric would be good.