performance: Async / non blocking producer sends #232

Given that the producer instance is shared a - batching levels should be excellent, and blocking on producer sends doesn’t slow anything else down, this would be a minor speed improvement.

However - for the vertx and reactor modules, this is an issue as it blocks the engine threads.

Callback should place result in controllers message queue for handling in next loop, which it then marks the work as complete.
Actually this could potentially just mark the work as complete directly without involving the controller thread.

This should actually be relatively easily to implement.