Fix memory leak of ItemRetrievalJobFactory in ItemRetrievalManager

Summary:
It was never deleted. Fixed by making the ItemRetrievalManager own
the ItemRetrievalJobFactory in all cases.

Test Plan: ctest -R AkonadiServer-itemretrievertest in an ASAN build

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21120