p2p/discover: improved node revalidation

[ucwong]

Node discovery periodically revalidates the nodes in its table by sending PING, checking if they are still alive. I recently noticed some issues with the implementation of this process, which can cause strange results such as nodes dropping unexpectedly, certain nodes not getting revalidated often enough, and bad results being returned to incoming FINDNODE queries.

In this change, the revalidation process is improved with the following logic:

• We maintain two 'revalidation lists' containing the table nodes, named 'fast' and 'slow'.
• The process chooses random nodes from each list on a randomized interval, the interval being faster for the 'fast' list, and performs revalidation for the chosen node.
• Whenever a node is newly inserted into the table, it goes into the 'fast' list. Once validation passes, it transfers to the 'slow' list. If a request fails, or the node changes endpoint, it transfers back into 'fast'.
• livenessChecks is incremented by one for successful checks. Unlike the old implementation, we will not drop the node on the first failing check. We instead quickly decay the livenessChecks give it another chance.
• Order of nodes in bucket doesn't matter anymore.

I am also adding a debug API endpoint to dump the node table content.