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.

node/node_test.go

@@ -454,9 +454,9 @@ func TestProtocolGather(t *testing.T) {
 		Count int
 		Maker InstrumentingWrapper
 	}{
-		"Zero Protocols":  {0, InstrumentedServiceMakerA},
-		"Single Protocol": {1, InstrumentedServiceMakerB},
-		"Many Protocols":  {25, InstrumentedServiceMakerC},
+		"zero": {0, InstrumentedServiceMakerA},
+		"one":  {1, InstrumentedServiceMakerB},
+		"many": {10, InstrumentedServiceMakerC},
 	}
 	for id, config := range services {
 		protocols := make([]p2p.Protocol, config.Count)
@@ -480,7 +480,7 @@ func TestProtocolGather(t *testing.T) {
 	defer stack.Stop()
 
 	protocols := stack.Server().Protocols
-	if len(protocols) != 26 {
+	if len(protocols) != 11 {
 		t.Fatalf("mismatching number of protocols launched: have %d, want %d", len(protocols), 26)
 	}
 	for id, config := range services {