SFU Migration support

[DanielNovak]

SFU migration support. The SFUs can send a GoAway socket event and in that case we need to migrate from one SFU to other. We spin up a second WS client connection to the new SFU’s signal server and negotiate with the same, already established RTCPeerConnection for the publisher with ICE restart, but create a new Subscriber PeerConnection and close the previous one.

We fall back to full reconnect if anything fails.

How to test locally:

• start two SFUs locally
• edit the code to use the local SFU address and switch to the second SFU address when GoAway is received
• connect two participants (two emulators)
• kill the first SFU (use Control-C, SIGTERM or SIGINT but not SIGKILL)
• validate that both participants migrated

TODO:

• live test with staging SFUs