Added and tested the network queue in the server

This is a reimplementation of the old network queue, but using a class.
This still uses a separate thread, so that packets can wait if there's any
lag. Really, thinking about it, I wonder how necessary this was.

On the upside, no singletons this time. Which means that you can have
several instances of UDPNetworkManager. That's unintentional, but good to
know.