Restrict duration of processing a Workflow to X minutes

[dr0i]

Playground is only that: a playground. It should not be used to do large ETL processing.
A generic approach to restrict Playground is to allow only so much processing time.
Processes that take longer (say: 5 minutes) should be aborted with a proper message.

This would also solve #159 in a way.

[dr0i]

AI didn't solve the problem, as the commits don't work.
Assigning @fsteeg .