Use extended types for sequence number and time stamp in RTPStats. (#1999)

Reinstating this PR - https://github.com/livekit/livekit/pull/1548

Will use extended types in other places too over time.

pkg/sfu/buffer/buffer_test.go

@@ -212,8 +212,8 @@ func TestNewBuffer(t *testing.T) {
 				buf, _ := p.Marshal()
 				_, _ = buff.Write(buf)
 			}
-			require.Equal(t, uint16(1), buff.rtpStats.cycles)
-			require.Equal(t, uint16(2), buff.rtpStats.highestSN)
+			require.Equal(t, uint16(2), buff.rtpStats.sequenceNumber.GetHighest())
+			require.Equal(t, uint32(65536+2), buff.rtpStats.sequenceNumber.GetExtendedHighest())
 		})
 	}
 }