[digiKam] Rewrite database KIO-slaves as pure Qt5 with multithreading

by Mohamed Anwer for KDE

Originally, KIO-Slaves was implemented to run database queries in a separate process to prevent problems with SQlite. Since SQlite support queries from separate threads, KIO-slaves can be dropped, and a new API can be implemented using pure Qt threads API. This will improve digiKam portability, and permit adjusting CPU cores assigned to database processes efficiently.