KIO: Task should be used in QVector, not QList.

Summary:
It's a struct with int+QByteArray so it's bigger than a pointer.

Minor effect on the overall numbers, it was only showing up
in the "allocations of a small size" list in heaptrack, so this is
more about not wasting space more than anything else.

Test Plan: jobtest

Reviewers: mwolff

Reviewed By: mwolff

Subscribers: Frameworks

Differential Revision: https://phabricator.kde.org/D7530