Osu: Add tracking of local player statistics

[ribbanya]

In the legacy osu!.db, there is no way to see how many times you've played a map. The API only allows you to see the past 50 play counts. There should be a way to locally keep track of how many times you've played a difficulty, similar to how osu!.db stores the highest rank achieved and last time it was played.

I looked through the model snapshot but didn't find anything that would keep track of play count.

[peppy]

Stats tracking is eventually planned, but we are holding off making large database changes as the underlying systems are going to see some refactor first. Will rename this issue to track the more general task.