Discovery zero refresh timer

[mh0lt]

This fixes an issue where the mumbai testnet node struggle to find peers. Before this fix in general test peer numbers are typically around 20 in total between eth66, eth67 and eth68. For new peers some can struggle to find even a single peer after days of operation.

These are the numbers after 12 hours or running on a node which previously could not find any peers: eth66=13, eth67=76, eth68=91.

The root cause of this issue is the following:

• A significant number of mumbai peers around the boot node return network ids which are different from those currently available in the DHT
• The available nodes are all consequently busy and return 'too many peers' for long periods

These issues case a significant number of discovery timeouts, some of the queries will never receive a response.

This causes the discovery read loop to enter a channel deadlock - which means that no responses are processed, nor timeouts fired. This causes the discovery process in the node to stop. From then on it just re-requests handshakes from a relatively small number of peers.

This check in fixes this situation with the following changes:

• Remove the deadlock by running the timer in a separate go-routine so it can run independently of the main request processing.
• Allow the discovery process matcher to match on port if no id match can be established on initial ping. This allows subsequent node validation to proceed and if the node proves to be valid via the remainder of the look-up and handshake process it us used as a valid peer.
• Completely unsolicited responses, i.e. those which come from a completely unknown ip:port combination continue to be ignored.