This change clears up confusion around the two ways in which nodes
can be added to the table.
When a neighbors packet is received as a reply to findnode, the nodes
contained in the reply are added as 'seen' entries if sufficient space
is available.
When a ping is received and the endpoint verification has taken place,
the remote node is added as a 'verified' entry or moved to the front of
the bucket if present. This also updates the node's IP address and port
if they have changed.
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