Rate limiting for discovery v5 messages and/or packets

[kdeme]

Implement rate limits on the discovery messages / packets:

• Handshake: one could continuously do handshakes with new peers (or same ones). This would be expensive for both sides, but can have implications on filling of the routing table. (See also discussion here: Adjust approach of randomNodes to a more lookup based system for providing peers #271)
• Ping: The pong reply is relatively small and not expensive. But a handshake that goes before the ping might add the node to the routing table.
• Findnode: The same applies regarding the handshake. But additionally, a nodes reply can be big and much more expensive. So this should be limited.

Based on this we need to look where to apply limits best. General packets, or something more subtle, or both.

[kdeme]

Related NBC issue from audit: status-im/nimbus-eth2#1359