Lock user stats/grades for update

[richardscull]

In this PR we are going to add lock for user stats and grades during score processing. As both entries are doing increments/decrements, we need to make sure the entities being updates are up-to-date.

I originally wanted to use atomic updates, but in MySQL database there is no good way to update entity and fetch new results as a single query. Another problem is that you can't ask EF Core to handle the atomic update itself, so you need to provide the delta values to the new method, which looks very sloppy.

As per my assumption, any score processing update will be blocked by the beatmap retrieval. Si using lock here wouldn't hurt the performance in general.

Another changes:

• I've rolled back the new split implementation of tests workflow, since it wasn't really running the tests. bruh.
• We now apply redis leaderboard update only after the db transaction commit.
• Early return for the grades processor, if grades are equal for the promoted/demoted score and updated score.