Propagator: change order of destruction

Propagator: change order of destruction

The destructor of the PropagateItemJob will access the propagator's
_activeJobList. So the _rootJob needs to be destroyed before it.
Order of destruction is the reverse of the order of the members in
the class. So put it at the end so it can be destroyed first.

(This made TestSyncEngine::testDirDownloadWithError crash sometimes
in the master branch)