Create Proxy request handler

[dylanneild]

Once SelectableStream handling is done, create a proxy request handler that allows requests to a nio-http instance to be proxied onto another HTTP server.

As part of this, set the request handler to support multiple handlers per URI space. The idea here is that if multiple handlers are recognized at the same URI, the server can round robin them.

This would effectively turn nio-http into a quick load balancer.