mpdreload: Fix batch deletion when there are still loads pending.

The batch deletion did not take into account the possibility that
some or all of the playlist may still need to be loaded afterwards
(this happens when there are tracks at the end of the target playlist
which are not in the current play queue).

This resulted in erroneous operation.  Fix that up.

And separate the batch loading into a separate routine to hopefully
clarify what the various track indices are so future me understands.