Peers calling schedule design

[williamlsh]

The workflow for every two calling parties to establish a WebRTC connection.

• Two parties send their shared passphrase to signal server once connected through WebSocket.
• The signal server tries to execute string set on Redis with command SET <passphrase> "" NX upon each party request, only the fastest party request arrives will be successful which returns 1, the other 0.
• The signal server assigns the caller role to the party which executes previous step successfully, to the other callee role. And return back to their respective roles.
• Now each party gets their roles. The caller party first sends its SDP offer, once the callee party receives its offer through signal server, it sends its SDP answer back. Then following WebRTC ICE candidates exchanging. Until WebRTC connection establishs successfully.