Parent task keeps a vector of statuses (task_essentials) of its finished
children. When the children number is large - for example because we
have many tables and a child task is created for each table - we may hit
oversize allocation while adding a new child essentials to the vector.
Keep task_essentails of children in chunked_vector.
Fixes: #25040.
Closesscylladb/scylladb#25064
b5026edf49