RTCMediaStreamStats can be made obsolete

[henbos]

Now that things have been moved to "outbound-rtp" and "inbound-rtp", "track" stats makes less sense, and "stream" stats less so since its a list of trackIds.

We could replace the dictionary by streamIdentifiers on the "sender" and "receiver"

[henbos]

• Add streamIdentifiers to sender and receiver stats
• Move RTCMediaStreamStats to the obsolete section

[vr000m]

@karthikbr82 @lennart-csio would removing this impact any of our algorithms or diagnostics?

[henbos]

TPAC decision: remove "track" and "stream" stats in favor of "mid"; don't need to add streamIdentifiers because that is knowable by other means and not very relevant to stats (if someone want that that should be filed separately).