This PR enables running the new discv5 protocol in both LES client
and server mode. In client mode it mixes discv5 and dnsdisc iterators
(if both are enabled) and filters incoming ENRs for "les" tag and fork ID.
The old p2p/discv5 package and all references to it are removed.

Co-authored-by: Felix Lange <fjl@twurst.com>