producer.replaceTrack(): strange behavior

[ibc]

Actually, sender.replaceTrack() works randomly in aiortc (or that's what I see in my experiments). When it "works", it keeps sending RTP. However the remote (i.e. Chrome) cannot render the received RTP for ~10 seconds and gets frozen video instead (but it does receive RTP). Asking for a key frame via RTCP PLI does not make it work. After ~10 seconds, the receiver can render the video again.

Said that, the above happens if I replace a webcam track with another new webcam track. If I try to replace a webcam track with a file video track, the remote gets frozen video forever, and I think that there is no sending RTP anymore.

NOTE: Obviously this may have relationship with #7.

[ibc]

Let's handle this issue in #9 and aiortc/aiortc#304.