Song Scanning Refactor (YARC-Official#886)

+ Improvements in `readonly` usage on entries
+ Optimizations with the cache file
+ Optimizations with memory usage during scan

Assets/Script/Audio/Bass/BassStemMixer.cs

@@ -74,12 +74,12 @@ protected override int Play_Internal(bool restartBuffer)
         protected override void FadeIn_Internal(double maxVolume, double duration)
         {
             float scaled = (float) BassAudioManager.ExponentialVolume(maxVolume);
-            Bass.ChannelSlideAttribute(_mixerHandle, ChannelAttribute.Volume, scaled, (int) (duration * SongEntry.MILLISECOND_FACTOR));
+            Bass.ChannelSlideAttribute(_mixerHandle, ChannelAttribute.Volume, scaled, (int) (duration * SongMetadata.MILLISECOND_FACTOR));
         }
 
         protected override void FadeOut_Internal(double duration)
         {
-            Bass.ChannelSlideAttribute(_mixerHandle, ChannelAttribute.Volume, 0, (int) (duration * SongEntry.MILLISECOND_FACTOR));
+            Bass.ChannelSlideAttribute(_mixerHandle, ChannelAttribute.Volume, 0, (int) (duration * SongMetadata.MILLISECOND_FACTOR));
         }
 
         protected override int Pause_Internal()