p2p: Filter Discv4 dial candidates based on forkID

[cskiraly]

Our Discovery/v4 based dial source was returning nodes without checking the forkid in the ENR, leading to lots of useless dials.

This PR adds a filter on top of the discovery/v4 stream of discovered nodes, returning only those that have a compatible forkID.

More in detail:

• to check the ENR, we need to retrieve it from the node first using an extra Discovery/v4 query. We add AsyncFilterIterator, which can filter a stream of nodes using a blocking filter function, doing several async checks in parallel.
• we set the AsyncFilterIterator to use the RetrieveENR function as both filter (retrieve could time out) and a modifier of the Node.
• we apply another filter on forkID, as we do in Discovery/v5.
• finally, we simplfy the mixing of discovery sources by moving the Discovery/v4 source from the Server to the Backend, next to the Discovery/v5 source.

Note: This PR is to be rebased after merging #31588

[cskiraly]

• updated with simplified AsyncFilter design
• removed dependence on p2p: name dial sources to allow better diagnostics and stats #31588

I've kept the forkID based filter separate from the ENR resolver for code clarity. These could eventually be combined, but I think the code becomes less clear, and the difference in performance should be negligible.

[cskiraly]

rebased on current master

[fjl]

Turns out we already added the filter in #31944, so I guess this can be closed now.