Database storage

[Muspah]

Is your feature request related to a problem? Please describe.
Ex. I'm always frustrated when plugins store data in large, LARGE sets of YAML-files.

Describe the solution you'd like
A serious database solution, including some file-based implementation as default (H2 for example), and MySQL/Postgres/... as configurable options.

Describe alternatives you've considered
Won't consider.

Additional context
Will self-assign.

Started implementing H2 as default layer.
Flyway for migrations.
Implementing playerDB at first and PR when ready.