routing_table: complete connecting state resolution

After a connectivity change, the DHT would enter the "connecting" state in order
to try to resolve good nodes. However, the previous algorithm would
probabilistically increase time for completion each time a node would fail to
resolve. We now avoid this by prioritizing non-expired nodes in the process.
Also, a subtle error would sensibly stop the process for 30 minutes due to using
the Node::NODE_EXPIRE_TIME constant instead of Node::MAX_RESPONSE_TIME.