wasContactedRecently recent query checks was inverted.
Kill a stupid find_node bug.

Upon receiving a response for a find_node that contained "closer" nodes,
the DHT engine was always recursing into that, getting into a pointless and
endless search.

Only continue a find_node search if the routing table is dangerously empty.

Fixes #9.
dht_test: log the count of find_node queries
Be less aggressive about get_peers recursive search.

Since searching the routing table is slow, avoid it as much as possible by
batching local peer searches. The present solution isn't very effective yet.