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.

We were assigning the var, before the type was compared which
could result in accepting "audio" stream for "video" JitsiLocalTrack.

When source-add and source-remove for the same SSRC arrive just one after
another, SSRC will not be removed as it didn't make it to the remote description
yet.

Before the change we were constructing lines to be removed based on SSRC
parameters values supplied in the notification. We want the client to simply
remove all lines related to the SSRC being removed as sometimes things get
out of sync. For example when Chrome adds 'default' mslabel or when we'll be
doing lipsync stream merging.