Rename dht_bucket → dht_routing_table.

Because that is a far better name. It also inspires the right thing, given what is written down in the Kademlia paper.
Provide a driver for the routing table.

We need to test the routing table without invoking other parts of the system. So we provide a driver for the routing table in order to model its behaviour correctly. The driver process tracks state of the routing table, so we can verify this state transition in a stateful test.
Introduce a lot of simple commands for manipulating the routing table.