Only lookup cached track if it is dirty in BaseTrackCache.

Since BaseTrackCache::data is very hot (called multiple times per table
cell rendered in the library) it saves a lot of time to avoid the
TrackDAO lookup if the track is clean anyway.

Anecdotally scrolling feels much smoother with this change. Profiling
results seem to confirm that this is about 4.3 times faster on average
and the variance dropped by 70%.

Base:
Debug [Main]: Stat("BaseTrackCache::lookupCachedTrack","count=43843,sum=5.9563e+07ns,average=1358.55ns,min=473ns,max=25679ns,variance=352422ns^2,stddev=593.652ns")

Experiment:
Debug [Main]: Stat("BaseTrackCache::lookupCachedTrack","count=47587,sum=1.49477e+07ns,average=314.114ns,min=132ns,max=13064ns,variance=36194ns^2,stddev=190.247ns")
Remove a nonsense comment I wrote a long time ago :).