Task archiving and exporting can run simultaneously which leads to errors then

It seems like, that archiving and exporting a task can be started simultaneously, which then leads to errors in archiving: "Archive zip does not contain all files".
I think this is because of the different locking mechanism of the export. This should be evaluated and fixed then. Correlates to TRANSLATE-625.

The behaviour is currently, that a export can be started every time, regardless of the tasks state. This must remain.
The workflow is: exporting a task, directly after that the PM triggers the archiving. Here should be achieved, that archiving does not start until exporting (especial file operations) is finished.