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.
so new one goes from mid point to high, and old one, goes from low to mid point. right?