for non-durable topics add a configurable Policy to drop messages from slow consumers

e.g. for market data pricing we may wish to only allow say 1000 pending messages but discard old messages rather than have things back up. Similarly we may wish to do something like remove old price changes for the same stock and just keep around the latest prices.

So some kind of pluggable policy on the non-durable topic subscription would be a good idea, letting users customize this behaviour.

Maybe some FlowControlPolicy or something?