This should avoid any potential issues with intertwined transactions on the same connection while still allowing higher throughput when importing.
Moves star difficulty calculation entry-point to Beatmap, and sets star difficulty at the correct place for song select to display.
Note that this is a very naive approach and will result in file access exceptions. This will be fixed in a further commit.
They can now only be instantiated from BeatmapDatabase and are abstract (to avoid misuse).
Adds base classes for difficulty calculations, hooks them up with carousel container, and adds a port of the osu difficulty calculator.
Also makes the connection private.
Also fixes the broken IPC condition
Moves import code to BeatmapDatabase.