p2p/dnsdisc: rate limit resolving before checking cache

[fjl]

This makes the resolver rate limit apply regardless of whether the record is already
cached. It helps avoid hot-spinning in pathological cases like:

it, _ := client.NewIterator(...)
it = enode.Filter(it, func(*enode.Node) bool { return false })
for it.Next() {}

Fixes #22509

[holiman]

LGTM