Fix Memory leaks in KDb, improve utils

Summary:

• Make AutodeletedHash delete items if needed for clear(), erase() and insert(), add more docs
• Fix memory leak in PropertyKDbUtils::Property::insert(): reuse previous object if needed
• Fix memory leak in Json Trader: delete plugin loaders that are skipped
• Remember created driver instances (this allows to reuse drivers and fixes memory leaks)
• Driver manager: remove no longer needed plugin loaders (fixes memory leak)
• Driver manager: remove driver metadata objects on exit (fixes memory leak)

Test Plan: cd builddir; ctest

Reviewers: piggz, wicik, wkosowicz

Subscribers: Kexi-Devel-list

Projects: KDb, KEXI

Differential Revision: https://phabricator.kde.org/D915