_tasks is currently std::list<shared_ptr<compaction_task_executor>>, but
it has no role in keeping the instances alive, this is done by the
fibers which create the task (and pin a shared ptr instance).
This lends itself to an intrusive list, avoiding that extra
allocation upon push_back().
Using an intrusive list also makes it simpler and much cheaper (O(1) vs.
O(N)) to remove tasks from the _tasks list. This will be made use of in
the next patch.
Code using _task has to be updated because the value_type changes from
shared_ptr<compaction_task_executor> to compaction_task_executor&.
(cherry picked from commit e942c074f2)
d63b9efa7e