Mitigate freezing during differential reload by parallelizing differential reload and the pack-specific forced reload

[jxu]

Describe the bug When loading large beatmap packs the game is nearly unresponsive.

To Reproduce

1. Download a large beatmap pack, such as the default pack options that show up when you have no songs preinstalled
2. Wait until they are being loaded

Expected behavior Game runs as normal

Screenshots N/A

Desktop (please complete the following information):

• OS: Windows 8.1
• Version: 0.70.3

[poco0317]

This game doesnt have beatmaps and doesnt have beatmap packs so i dont know where you get that from.

But The game being frozen when loading a pack is "intentional."

Related: #388