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.
perhaps here we should make sure the `cur_range` >= constants.k ? so that we actually have something to split.