If std::vector is resized its iterators and references may
get invalidated. While task_manager::task::impl::_children's
iterators are avoided throughout the code, references to its
elements are being used.
Since children vector does not need random access to its
elements, change its type to std::list<foreign_task_ptr>, which
iterators and references aren't invalidated on element insertion.
Fixes: #16380.
Closesscylladb/scylladb#16381
(cherry picked from commit 9b9ea1193c)
65fb562ae3