Hostdb polish

I moved the HostEntry type to the hostdb package. I renamed a few functions, and changed the way a few variables work. I added some testing. I struggled a lot with rebaseing and merging, as you can see form the commit history.

Nothing really fundamental changed. I want to test more but will be holding off until we decide how the RPC stuff should work, and until we have a better idea of how all the modules are supposed to fit together.

I don't think that the module situation is very dire. I think what we have is very close to good and there are some corner cases (particularly around the gateway) that need to be reconsidered. Beyond that, I do really like what we have and don't think any more major changes to the fundamental design will be necessary.