WIP Almost complete refactoring and unit testing of routingtable. Minor refactoring of kbucket.

Very WIP. I will describe tomorrow; really tired now.

### Bugs found:
* Many operations are potentially thread-unsafe.
* The `replacement_cache` isn't synchronized once a `KBucket` is split.
The logic was fine, so long each `KBucket` was sovereign over its own ID space. I just felt there was a simpler way to write it. I also dropped the random ID generator, because it seemed pointless.