Currently, progress of a parent task depends on expected_total_workload,
expected_children_number, and children progresses. Basically, if total
workload is known or all children have already been created, progresses
of children are summed up. Otherwise binary progress is returned.
As a result, two tasks of the same type may return progress in different
units. If they are children of the same task and this parent gathers the
progress - it becomes meaningless.
Drop expected_children_number as we can't assume that children are able
to show their progresses.
Modify get_progress method - progress is calculated based on children
progresses. If expected_total_workload isn't specified, the total
progress of a task may grow. If expected_total_workload isn't specified
and no children are created, empty progress (0/0) is returned.
Fixes: https://github.com/scylladb/scylladb/issues/24650.
Closesscylladb/scylladb#25113
a7ee2bbbd8