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/node.go

@@ -98,6 +98,13 @@ func (n *Node) Pubkey() *ecdsa.PublicKey {
 	return &key
 }
 
+// Record returns the node's record. The return value is a copy and may
+// be modified by the caller.
+func (n *Node) Record() *enr.Record {
+	cpy := n.r
+	return &cpy
+}
+
 // checks whether n is a valid complete node.
 func (n *Node) ValidateComplete() error {
 	if n.Incomplete() {