Fix implementation regarding rpc.findNode (§4.3)

The recipient of a FIND_NODE should never return a triple containing the
nodeID of the requestor. If the requestor does receive such a triple, it
should discard it. A node must never put its own nodeID into a bucket as a
contact.
Fix bugs (lookup algorithm, Id serialization), add FIXMEs
Fix Id conversion (Buffer.prototype.write invocation issue)
iterativeFindValue: store value on closest contact if it didn't reply
FIXMEs and store contacts found on bootstrap
DHT: use iterativeFind* operations
Implement bucket refresh
Mark a bucket refreshed when performing an iterativeFind* operation