Add :Async option to detach connections and handle in separate actors

[chewi]

The change looks big but it's mostly just moving methods from the Server class to the new Handler class. I've only given it a quick try last thing on a Friday but it seems to work well. I could possibly add some tests next week if required.

[chewi]

I've realised I need to terminate the actors I'm creating on each request but adding terminate to the end of async_on_connection doesn't seem to do anything and even if I manually terminate actors from a console, they seem to still sit in the thread list. 😞

[chewi]

Never mind, I figured it out.