p2p: improve disconnect signaling at handshake time

As of this commit, p2p will disconnect nodes directly after the
encryption handshake if too many peer connections are active.
Errors in the protocol handshake packet are now handled more politely
by sending a disconnect packet before closing the connection.

p2p/handshake_test.go

@@ -143,7 +143,7 @@ func TestSetupConn(t *testing.T) {
 	done := make(chan struct{})
 	go func() {
 		defer close(done)
-		conn0, err := setupConn(fd0, prv0, hs0, node1)
+		conn0, err := setupConn(fd0, prv0, hs0, node1, false)
 		if err != nil {
 			t.Errorf("outbound side error: %v", err)
 			return
@@ -156,7 +156,7 @@ func TestSetupConn(t *testing.T) {
 		}
 	}()
 
-	conn1, err := setupConn(fd1, prv1, hs1, nil)
+	conn1, err := setupConn(fd1, prv1, hs1, nil, false)
 	if err != nil {
 		t.Fatalf("inbound side error: %v", err)
 	}