scanner: Switch to gst.pbutils.Discoverer and refactor API

- New API wraps gst.pbutils.Discoverer for mopidy use.
  - Custom scanner code removed.
- Callback based async API switched with simpler synchronous API
  - Scanner is feed one and one URI and returns the result or raises an error.
- Adds new ScannerError exception for error handling in scanner
- Limits local/scan_timeout to 1000-3600000 i.e. 1s to 1h
- Files with zero duration are now excluded.
- Mtime handling moved out of scanner so we can use it for streams etc.

Also fixes slowest tests found in #534 and paves the way for making the stream backend lookup metadata.