Hostdb

Bring back the host db, write a bunch of tests for it.

The race tests fail, but don't trigger race conditions. I'm exactly sure what's going on but I have suspicions that the race library is intentionally preventing the blocks from propagating in a particular order, because I only enforce that by calling Sleep().

Sleep() is being called to simulate the passage of time in between two nodes in a real world environment, and is not being used to enforce actual synchrony. Ah well.