Add an ParallelStep or AsyncStep feature (optionally: Tasks)

[jochenwezel]

Purpose

• allow running multiple steps in parallel (async)
• optionally: allow running multiple tasks in parallel (async)

Design challenges

• handle exceptions in 1 step and define correct behaviour in parallel steps
• rollback/rollforward which steps after exception in 1 parallel step
• user cancel event: correct rollback/rollforward behaviour?
• correct rollback/hard-fail behaviour in critical and non-critical tasks/steps