audio: Update scanner to not use gst.Bus.poll

Turns out poll sets up it's own mainloop in the default context causing us to
segfault.
audio: Improve audio_data_to_track handling.

- Handle missing or none data for duration and mtime
- Add organization, location and copyright mapping used for streams.
streaming: Add scanner lookup of stream metadata.

This adds support for looking up metada for all any any protocols the streaming
backend will support. This should also ensure that file:// files get metadata.
docs: Add streaming metadata lookup to changelog
docs: Add stream/timeout config value to docs
streaming: Code review adjustments
Merge pull request #603 from adamcik/feature/lookup-stream-metadata

Add metadata lookup to streaming backend.