- just a hotfix, a proper solution is to get rid of zone_contents->zone link, it's not really needed.

refs #187

The purpose is to observe the behavior of 1.4.1 compared to 1.4.0
refs #187

Resizing of thread units on the run is not used nor endorsed,
as it's hard to guess when the specific threads terminate or not.
(They are allowed to run to the nearest cancellation point or finish.)

Closes #184