Handling currentBroadcastStartTime

PLAT-1762
@tan-tan-kanarek Agreed that it's not symmetrical/balanced. However, this measurement needs to be as accurate as possible, and there's no immediate event that is sent from the media-server to the server upon onPublish.. yet. There WILL be one after we implement the BROADCASTING/PLAYABLE/VOD events, and then we'll be able to shift currentBroadcastStartTime  to the server side code.

Regarding reset, the reason to do it on the server side, upon unregister, is to ensure cleanups even if the media-server has crashed (via the fake unregister).