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/enode/nodedb_test.go

@@ -332,7 +332,7 @@ var nodeDBExpirationNodes = []struct {
 			30303,
 			30303,
 		),
-		pong: time.Now().Add(-nodeDBNodeExpiration + time.Minute),
+		pong: time.Now().Add(-dbNodeExpiration + time.Minute),
 		exp:  false,
 	}, {
 		node: NewV4(
@@ -341,7 +341,7 @@ var nodeDBExpirationNodes = []struct {
 			30303,
 			30303,
 		),
-		pong: time.Now().Add(-nodeDBNodeExpiration - time.Minute),
+		pong: time.Now().Add(-dbNodeExpiration - time.Minute),
 		exp:  true,
 	},
 }