use the same MediaStreamTrack on RTCRtpSender

[jerry2013]

this took me lots of time to track down.

under iosrtc, for local media tracks, the normal flow goes:

• app calls gUM
  • iosrtc plugin sends intent/request to native side
  • webrtc does things
  • native side sends OK event to js, containing the track info
  • iosrtc plugin returns to the caller with a new MediaStreamTrack
• app tries to add the track in the context of peer-connection/rtp-sender (new sender or replaceTrack)
  • iosrtc plugin sends intent/request to native side
  • webrtc does things
  • native side sends OK event to js
  • iosrtc plugin should directly assign the existing track to the peer-connection/rtp-sender
    • but, since the track is not yet on pc, pc.getOrCreateTrack then makes new MediaStreamTrack with the same id
    • this breaks the spec since the track from gUM is not "object reference equal" to the track linked to rtp-sender

the patch here uses a global map to maintain all "active" tracks, so that the async callback can find the existing tracks.

still, this is a quick and dirty workaround to addresses my own usage.