Concurrency - Lambda based message passing

[xoft]

Unless there's a lot of potential in the rewrite, I'm against it - too much of a change for very little benefit. The performance is already good enough, and the deadlocks happen in other parts of the code. Not sure about the tidier part either, now it's tidy because it's wrapped into functions with clear names and the implementation details (such as moving it to a queue for later processing) is hidden from the client classes.