Possible VoIP stack improvements

[SimonBrandner]

This is an issue where @SimonBrandner will complain about our VoIP stack and suggest things we should do/consider doing differently

---

• We should be using tranceivers instead of senders and storing those
  • They are more accurate about trackIds
  • They can give you mids which
  • Both points above will be important for SFUs
• We shouldn't be storing senders on MatrixCall and we should instead be putting them on CallFeeds
  • The current way isn't flexible at all
• We shouldn't have usermediaFeeds and screenshareFeeds on MatrixGroupCall
  • We should be always able to concatenate CallFeed arrays from individual calls (note: performance?)
  • It's painful having to figure out what CallFeeds to add and which ones should go
• We should never assume the user will only ever want to send N CallFeeds of this purpose and M CallFeeds of that purpose
  • We should be more flexible
• It doesn't feel like a good pattern to separate CallFeeds by purpose in the non-user facing code (Šimon looks at MatrixGroupCall)
  • We might want to allow custom purposes
• We should consider not using negotiationNeeded event and we should do things more manually
  • This might be necessary for the SFU