FREEDB-11: try matching without data track

A data track is part of a FreeDB TOC, but not a MB TOC.
So when we don't find anything,
we try to match without the last track,
which guessed as being a data track.
import Errors from sqlalchemy
use normal and data track durations in one DB query

We shouldn't issue more queries than we need to.
remove toc_rows2 again
fix 1-track TOCs

For these durations2 is [], which wouldn't work with our calculations
and also not for the DB query.
We just copy durations, which translates to OR false in the query,
because there are no releases with num_tracks 0.
add comments about types of ids given

Since I spent quite some time, why completely misleading
freedb ids are returned, I had to add comments in the code.

misc "freedb ids" are no valid freedb ids, but MB medium ids.
These are actual unique ids for the MB database
but don't follow the XXYYYYZZ schema.

On freedb collisions (rock, real freedb id) only one result can be given.
Merge pull request #1 from JonnyJD/data_track

FREEDB-11: try matching without data track