clarify limitations on running import/export tasks concurrently

Product team is expecting that that multiple Import and Export tasks cannot run concurrently. Instead the claim is that one may be actually doing something, while all others may indicate they are running in the UI, but are actually blocked in the background.

However we have noticed two behaviours that contradict this claim.

Scenario 1: The tasks UI shows multiple import tasks running, instead of one running and the others in Blocked state.

Scenario 2: A thread dump proves that the tasks actually are running concurrently, on file operations. See threads.txt where 9 of these tasks are running and basically stuck on the slow file system/ network mount source dir.

Expected

Document if tasks can run concurrently or not and provide recommendations of doing this with regards to performance concerns and best practice.

OR

impose some sort of programatic limitation on concurrently running these types of tasks ( like one per repo, per format ) as it sounds like product expected only one to be run at a time.