cmd/devp2p, p2p: dial using node iterator, discovery crawler (#20132)

* p2p/enode: add Iterator and associated utilities

* p2p/discover: add RandomNodes iterator

* p2p: dial using iterator

* cmd/devp2p: add discv4 crawler

* cmd/devp2p: WIP nodeset filter

* cmd/devp2p: fixup lesFilter

* core/forkid: add NewStaticFilter

* cmd/devp2p: make -eth-network filter actually work

* cmd/devp2p: improve crawl timestamp handling

* cmd/devp2p: fix typo

* p2p/enode: fix comment typos

* p2p/discover: fix comment typos

* p2p/discover: rename lookup.next to 'advance'

* p2p: lower discovery mixer timeout

* p2p/enode: implement dynamic FairMix timeouts

* cmd/devp2p: add ropsten support in -eth-network filter

* cmd/devp2p: tweak crawler log message

p2p/protocol.go

@@ -54,6 +54,11 @@ type Protocol struct {
 	// but returns nil, it is assumed that the protocol handshake is still running.
 	PeerInfo func(id enode.ID) interface{}
 
+	// DialCandidates, if non-nil, is a way to tell Server about protocol-specific nodes
+	// that should be dialed. The server continuously reads nodes from the iterator and
+	// attempts to create connections to them.
+	DialCandidates enode.Iterator
+
 	// Attributes contains protocol specific information for the node record.
 	Attributes []enr.Entry
 }