Streaming API for existing on-disk storage

[ikatson]

I was playing around yesterday to see how we would go about implementing #110

Although a full stream-from-memory would require a very different API, and re-assembling components in a novel way, I first tried to make an API that would stream an existing file from disk and:

• block the stream unless pieces are available
• prioritize streamed pieces

This made it possible to play those URLs in VLC and "ffplay", and even seek, jump around.

This might be useful if you are planning to store the file eventually anyway.

As for streaming from memory, will try to do sometime later.

This one isn't complete yet though:

• need a bit more polish and testing. Maybe simplification.
• it doesn't stream the file unless it's checked for streaming
• need to wake up peers that are waiting for things to do if we start/seek a stream