tag_scan: Fix painful rescan of music metadata on startup.

For the longest time, JuK has suffered from a problem where its intended
behavior to load music metadata from a cached database, instead of
re-scanning every individual track on startup, was not working right.

There has been debugging lines in JuK since all the way back to 2013
trying to trace what area of startup sequence was taking up all the
time, but to no avail in helping me fix the bug.

The Problem
===========

Recently I took a different approach, of adding a debug-only crash
whenever we would load a music track tag the "slow" way, and long story
short there were two bugs that each would cause slowdown:

1. Playlists aside from the CollectionList would cause every music track
   in that playlist to be re-scanned. What this means is that every
   though the music in the CollectionList would be loaded quickly, if
   you had that same music track in a separate Playlist, that music
   track would reload the same tags from disk rather than cop...