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_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.