routing: integrate probe state machine

[iand]

Integrates the probe state machine from probe-lab/go-kademlia#111 as part of probe-lab/go-kademlia#45

The include state machine is responsible for checking and adding nodes to the routing table. The probe state machine is responsible for checking and potentially removing nodes from the routing table. They operate independently but there is some cross-notification of events.

When the include state machine has performed an initial check it emits a StateIncludeRoutingUpdated state. This is used to notify the probe state machine that a new regular connectivity check must be created.

When the probe state machine fails a check for a node it emits a StateProbeNodeFailure state. This is used to notify the include state machine to add the failed node as a candidate for inclusion as described in probe-lab/go-kademlia#45. This effectively gives the node a second chance, if it fails the inclusion check it is dropped completely.