Create new Xapian DBs in Akonadi datadir instead of Baloo

When creating a new Xapian databases (for new user for instance), create them
in Akonadi datadir (i.e. XDG_DATA_HOME/akonadi/search_db) instead of Baloo.

This does not affect existing users - the code will still look into Baloo datadir
first if the databases exist there and use them. There is no migration to move
the databases due to concurrency issues (there are multiple readers starting in
undeterministic order so it's not clear who should perform the migration in order
no to break the other.