Discovery: additional security check before sending Neighbours

[gumb0]

Before answering FindNode we should also check that it came from the same IP address as Pong

(otherwise it's possible to send valid Pong and then send FindNode with the IP of the node you want to do amplification attack on)

[gumb0]

My comment from #5434 :

I've been thinking about the case of the changed endpoint and its relation to the vulnerability #5455, and now it seems to me that we handle it insecurely - currently if endpoint changes we continue to trust this node's endpoint proof without re-pinging it.

I think we should

• remove updating the endpoint from noteActiveNode, probably just remove _endpoint parameter. This function will assume that node's enpdoint is up-to-date.
• make addNode in case endpoint changes invalidate pong sent/received times (set to 0) and send another Ping

• also probably remove the current record with the old endpoint from the node table bucket?