Add max_recursion check to task execution.

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In order to prevent unintentional infinite loops, add a special header / option value called max_recursion_depth and current_depth that gets automatically set and incremented when tasks are inserted.

Inserting an async task from an async should result in the current_depth being incremented by one. There should also be a mechanism to set max_recursion_depth and to reset current_depth, so that users can override the defaults if needed.

If current_depth reaches max_recursion_depth, the task should automatically abort (see enhancement #38).

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This has been merged to master.