lnwire+peer: clamp pong bytes, make ping handler more effcieint

This was not properly enforced and would be a spec violation on the
peer's end. Also re-use a pong buffer to save on heap allocations if
there are a lot of peers. The pong buffer is only read from, so this
is concurrent safe.

peer/brontide.go

@@ -327,6 +327,12 @@ type Config struct {
 	// from the peer.
 	HandleCustomMessage func(peer [33]byte, msg *lnwire.Custom) error
 
+	// PongBuf is a slice we'll reuse instead of allocating memory on the
+	// heap. Since only reads will occur and no writes, there is no need
+	// for any synchronization primitives. As a result, it's safe to share
+	// this across multiple Peer struct instances.
+	PongBuf []byte
+
 	// Quit is the server's quit channel. If this is closed, we halt operation.
 	Quit chan struct{}
 }
@@ -1394,10 +1400,8 @@ out:
 
 			// Next, we'll send over the amount of specified pong
 			// bytes.
-			//
-			// TODO(roasbeef): read out from pong scratch instead?
-			pongBytes := make([]byte, msg.NumPongBytes)
-			p.queueMsg(lnwire.NewPong(pongBytes), nil)
+			pong := lnwire.NewPong(p.cfg.PongBuf[0:msg.NumPongBytes])
+			p.queueMsg(pong, nil)
 
 		case *lnwire.OpenChannel,
 			*lnwire.AcceptChannel,

peer/brontide_test.go

@@ -999,6 +999,7 @@ func TestStaticRemoteDowngrade(t *testing.T) {
 					Features:       test.features,
 					Conn:           mockConn,
 					WritePool:      writePool,
+					PongBuf:        make([]byte, lnwire.MaxPongBytes),
 				},
 			}
 
@@ -1103,6 +1104,7 @@ func TestPeerCustomMessage(t *testing.T) {
 			}
 			return nil
 		},
+		PongBuf: make([]byte, lnwire.MaxPongBytes),
 	})
 
 	// Set up the init sequence.