net/livemedia: update to 2020.01.10

2020.01.10:

• Made sure that each "TaskToken" is set to NULL when its scheduled event occurs, to ensure that "unscheduleDelayedTask()" is never later called with it. Also, because "unscheduleDelayedTask()" sets its "TaskToken&" parameter to NULL, there's no need to do this explicitly outside the call. (Thanks to Helmut Grohne for proposing this.)

2020.01.09:

• Improved the "MediaSession"/"MediaSubsession" SDP parsing code by moving replicated code into a single function. (We also now parse the "key-mgmt" SDP attribute, although we don't yet use it.)