Make sfu-to-sfu talk MSC3401 and add new features

[ara4n]

Fixes #13
Implements MSC3401 SFU bits
Requires matrix-org/matrix-js-sdk#2423
Requires mautrix/go#77

[SimonBrandner]

This now looks like an idea much worse than when I first thought of it... This could definitely be done differently now though I think it makes sense to just try to redo this during switching to using a pool of streams since that might change the design again...

[dbkr]

Generally looking good. Main things are proper (or at least some) error handling rather than panicking and making sure member functions aren't public unnecessarily. We'll also want to hook up logrus or whatever people use now at some point soon, but probably not in this PR.

[SimonBrandner]

We'll also want to hook up logrus or whatever people use now at some point soon, but probably not in this PR.

Yep, that has been my thoughts since the fist time I actually need to debug it based on logs :P

Sorry, for the Go newbie mistakes, hopefully, it looks better now

[dbkr]

Looks good other than the ice password. Presumably we're blocked on the mautrix-go PR now?

[SimonBrandner]

Looks good other than the ice password.

Thank you!

Presumably we're blocked on the mautrix-go PR now?

Since I expect some of the events to change shape with further development, I'd hold off and wait and get that merged when we're more sure things are not going to change radically