Since (1) half of the disco uses check whether the feature in question
is supported, (2) the other half appears to not absolutely require
disco, and (3) disco needs to be explicitly included/required in order
to be supported, make disco completely optional.

While we have JitsiMeetJS defined as a global variable in Jitsi Meet, it
doesn't sound like an absolutely necessary and beautiful requirement
inside the library itself.

Rename variables with "stream" in name which refer to JitsiTrack.
Rename TRACK ADDED/REMOVED events.
Try to reduce direct access to remoteTracks object.
Log warning message when track is overwritten.

move sessionId reading/storing into Settings.js