Implement proper sorting of Postgres versioned directories

Summary:
Until now we had a special hack for 10.x series, but with the release of
Postgres 11, we need a more generic solution.

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22274