Replace the lamps' mob-killing logic with mob-blocking logic.

[Baughn]

This should be far more efficient, performing a logarithmic-time operation on every mob spawn -- once. Instead of having each and every lamp search for mobs. It's also more likely to work.

• [ ] Test thoroughly.
• [ ] Find a better data structure, and make this work outside of the 32km by 32km range it has now.