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.
Right. The ranges are [half-open](http://stackoverflow.com/questions/13066884/what-is-half-open-range-and-off-the-end-value).