libtorc-core: Improved http/s streaming

- use byte range streaming if available from the http server
- support peek and seek operations for streamed downloads
- rename some member vars in TorcNetworkRequest to make them a little
clearer
- reserve of a portion of the streaming ringbuffer for 'old' data.
- allow seeks within the buffered region and 'old' data region
- start a new request for anything outside of this range

Handles playing and seeking various awkward container formats that seek
continuously within the stream.