gcc 11.1 on my arch system complains that the calls to free
does not match the allocator function new[].

This introduces AbstractWorkStealingScheduler which holds the common
work-stealing scheduling strategy.

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.