Implement LiveStreaming Rewind Buffer

* On slave streams, `rewind.hls_segmenter` is a buffer for HTTP Live Streaming
  segments. We segment once, and then have an array of buffers that we can give
  out to live streaming clients.
* Split `Rewinder` and `HLSSegmenter` out into their own files
* Retool the `Rewinder` setup process so that we deliver pump information in the
  callback instead of in a later `pump` event.