p2p, p2p/discover: add signed ENR generation (#17753)

This PR adds enode.LocalNode and integrates it into the p2p
subsystem. This new object is the keeper of the local node
record. For now, a new version of the record is produced every
time the client restarts. We'll make it smarter to avoid that in
the future.

There are a couple of other changes in this commit: discovery now
waits for all of its goroutines at shutdown and the p2p server
now closes the node database after discovery has shut down. This
fixes a leveldb crash in tests. p2p server startup is faster
because it doesn't need to wait for the external IP query
anymore.

p2p/server_test.go

@@ -225,12 +225,15 @@ func TestServerTaskScheduling(t *testing.T) {
 
 	// The Server in this test isn't actually running
 	// because we're only interested in what run does.
+	db, _ := enode.OpenDB("")
 	srv := &Server{
-		Config:  Config{MaxPeers: 10},
-		quit:    make(chan struct{}),
-		ntab:    fakeTable{},
-		running: true,
-		log:     log.New(),
+		Config:    Config{MaxPeers: 10},
+		localnode: enode.NewLocalNode(db, newkey()),
+		nodedb:    db,
+		quit:      make(chan struct{}),
+		ntab:      fakeTable{},
+		running:   true,
+		log:       log.New(),
 	}
 	srv.loopWG.Add(1)
 	go func() {
@@ -271,11 +274,14 @@ func TestServerManyTasks(t *testing.T) {
 	}
 
 	var (
-		srv = &Server{
-			quit:    make(chan struct{}),
-			ntab:    fakeTable{},
-			running: true,
-			log:     log.New(),
+		db, _ = enode.OpenDB("")
+		srv   = &Server{
+			quit:      make(chan struct{}),
+			localnode: enode.NewLocalNode(db, newkey()),
+			nodedb:    db,
+			ntab:      fakeTable{},
+			running:   true,
+			log:       log.New(),
 		}
 		done       = make(chan *testTask)
 		start, end = 0, 0