Calculate AcoustID of the tracks in import process
This would allow us to do loads of things after that even if track doesn't have musicbrainz ID.
The examples of possibilities after this is implemented:
- extension of !1394 (closed) / #1218 to tracks not having mbid
- uploading AcoustID for tracks with mbid to the upstream
- automatic grabbing of track's missing metadata and mbid if wanted
- automatic genre categorization