Implementing enemy spawn regions on the server

This check-in is mostly focused on code to support enemy spawn regions
server-side. I've stripped most of the code from the client except the
region definition itself, and I've created a new interface called
IUpdateable for objects that need to be updated every server tick.

As of right now, enemy spawn regions are behaving the same as they did
on the client, just on the server now. Next up - enemy AI porting! And
eventually need to connect the enemy spawning with network messages.