* Player data to Database
Add player data into databases (SQLite3 & PG only)
PostgreSQL & SQLite: better POO Design for databases
Add --migrate-players argument to server + deprecation warning
* Remove players directory if empty
* Remove the copy from db::loadBlock by using a pointer to the destination
* cleanup db backend, the child backend doesn't have to set their functions as virtual
This allows other applications (e.g. minetestmapper) to interrogate
the database while minetest is running, without causing an almost
certain minetest crash.