Endless loading-loop in task-properties panel under certain circumstances

As mentioned in https://jira.translate5.net/browse/TS-2280 under certain circumstances the task-properties panel can get in a state, where it loads the task-properites in an endless loop with a frequency of approx 2-3 requests / second. This phenomenon might added greatly to the server non responding anymore as experienced in TS-2280. As a sidenote, the effect even happened, when the task-panel was invisible.

Cause:

The cause seems to be, that the 100% progress triggers a load of the task-properties sub-panels. If the server responds to slow, the next 100% progress again triggers the loading which overwhelms in sum the server and leads to an endless loop of unsuccessful loads.

Solution:

• Setting the task-progress (via the messagebus) must be an atomic operation that triggers no other events
• There must be an explicit, singular event "import ended" that triggers loading the task properties when the import ends - either with setting the task to "open" or "error"
• When a load is running for a store, triggering a new load must only be initiated, if a param changed. Otherwise a redundant load should lead to a silent ignore

Replicating this effect can be attempted with the help of google's dev-tools request speed throtteling, although I don't know if this is enough to trigger it.