Impact on Track Stats because of moving stats from sender/receiver to appropriate in/outbound rtp

[vr000m]

Related #402.

[vr000m]

@pallab-gain @karthikbr82 could you make sure that the tracks have the right stats once @henbos makes the appropriate changes to RTC[In/Out]boundRtpStreamStats

[vr000m]

We probably to discuss replaceTrack and onstatsended but also to discuss if the trackattachment stats are needed and instead rely on track stats.

cc: @henbos @pallab-gain @karthikbr82

[henbos]

If "onstatsended" fires for the track attachment we could do getStats() to get outbound-rtp values at the time of the event, though it wouldn't be a perfect snapshot because the counters in the outbound-rtp might keep increasing.

[henbos]

Let's discuss further when #463 has landed

[henbos]

#463 has landed

[henbos]

I just filed #479 arguing for the removal of these dictionaries. I think "what to do about onstatsended?" should be discussed in a separate issue, so I filed #480.

With those two issues, I think we can close this issue as having been split up into two issues. Feel free to object.