Update for KDb 3.0.91: Improve memory management of raw SQL execution and preparing

Summary:
Depends on https://phabricator.kde.org/D5771

FIXED-IN:3.1.0

Test Plan: build and try Kexi with sqlite/pgsql/mysql projects.

Reviewers: piggz

Tags: KEXI

Differential Revision: https://phabricator.kde.org/D5772