Abstracting user data storage logic

[maxcom1]

Hey! Is it possible that in some time the user data saving/reading system will be abstracted? The current system could remain as a default option, and plugins could create their own load/save implementation (to avoid adding an extra thing for EssentialsX to maintain).

I don't mean anything in the direction of synchronization between servers, just the current mechanism of creating tens of thousands of separate files for each player is quite ancient at this time and problematic even for small servers, and if we want to strive for EssentialsX to be a little bit modern and keeping up, I think it's a thing worth considering.