parallellize enqueue

Enqueue is slow because it calls libraries to read Flac and mp3
metadata.  Parallellizing it gives very acceptable speedup:

        Enqueued 13513 tracks in 994 directories (0 streams).
        riddim -e *  34.06s user 34.64s system 272% cpu 25.206 total

This can always be improved, but it was a multi-minute operation
previously.