Fix aggregation logic for cacheOnly and ignoreErrors.
Summary:
Add unittests for AggregatedFetch*Scope.
They show that subscribers really need to better clean up after
themselves when unregistering, that's for the next commit (this test is too
low-level for that).
Test Plan: new test
Reviewers: dvratil
Reviewed By: dvratil
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D18670