A threadpool controls a number of threads that execute a number
of thread *tasks* from a queue. The API even says they are tasks.
So it's damn confusing when ZipPackageStream::saveChild()
claims to limit the number of threads to 4x the maximum number
of threads. It limits the number of queued thread tasks.

Change-Id: I317497f27a82d92a7c8566b14aaeae73a4ffef1f
Reviewed-on: https://gerrit.libreoffice.org/72677
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>