Make running/waiting for multiple subtasks more flexible and convenient

[Gjum]

When a task wants to start several subtasks and wait until one or all of them are finished, a lot of boilerplate is needed.

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"all"

• A waits for B and C
• C finishes first, A keeps waiting
• B finishes, A continues

  "any"

• A waits for B or C
• C finishes first, A continues, B is aborted

  lock management

• if A has the lock, B and C get the chance to acquire it as their first action
• if A had the lock before waiting, it should automatically be acquired again before A continues