Transcoding: Add initial handling for discontinuities

* FrameChunker: Add `resetTime` function, for marking that the next chunk should
  be given a non-contiguous timecode
* TranscodingSource: Add code to watch for source interruptions. Reset chunking
  timecode if data stops for more than 30 seconds.
* FIXME: We aren't currently doing anything about the resulting discontinuities
  in the HLS playlist output. We need to do so in order to conform to the spec.