Make running/waiting for multiple subtasks more flexible and convenient #151

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

"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