Those tasks might not have finished yet, so there's a small race when
canceling and restarting a task.

Change-Id: I9f9ffcc6869455e7d8a407ff6ff4c40518d60f27
Task-Nr: QTCREATORBUG-4801
Reviewed-on: http://codereview.qt.nokia.com/419


Reviewed-by: Daniel Teske <daniel.teske@nokia.com>