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Currently it is easy to deadlock wcfs by opening .wcfs/zhead and not
reading from opened zhead handle due to the nature of what this zhead
file is designed to do. For this reason we want .wcfs/zhead not to be present
on production deployments and be available only where it is needed -
during test runs.

-> Introduce -debug flag for that to "enable debugging features" and put
   .wcfs/zhead to be that debug-only feature, and move it to be
   .wcfs/debug/zhead for clarity.

/reviewed-by @levin.zimmermann
/reviewed-on !36