We are unable to switch to C++20 until
https://github.com/mesonbuild/meson/issues/8084 is fixed.

In debug builds, the schedule() method is now guarded by an assert(),
as due to this change, the schedule() method will no longer work from
everywhere.

This also improves the worker sleep method.

And fix the #include issues reported by the newer IWYU version.

This should not change much, but ensure that this method is also
invoked in the test (even though it is not explicitly tested).

The CPS already gets incremented by the
SynchronizedFiber. Initializing the CPS with FIBER_COUNT means that
the CPS will never become ready.