Add single-player optimizations

[patowen]

When playing Hypermine in single-player, there are a few things that it does that do not make sense from an outsider's perspective:

• In Windows, the first time you play it, a Windows Firewall message pops up. This could be prevented by simulating network features by doing everything in memory instead of sending TCP/UDP packets through localhost.
• Both the server and client generate chunks independently, which, in single player, means that many chunks are generated twice unnecessarily.
• The server and client's steps are timed independently, meaning that one can lag without the other, causing difficult-to-justify jerky movement.

Ideally, it should be possible to resolve the above issues without significantly increasing maintenance burden.

I don't think it makes sense to prioritize this issue until enough gameplay features are fleshed out that the codebase has stabilized somewhat, and render-distance related optimizations have been improved. This is especially true because doing this early risks single-player and multi-player code diverging, making testing netcode difficult (and easy to forget).