multiprocessing.Pool.close() ensures no new tasks can be submitted
to the pool and waits for them to all finish. Even though
AsyncResult.get() also waits for the tasks to finish, and our code
structure shouldnt submit new tasks at that point, close() first,
get() then. In the future this could be error-prone in the future
where mp_tasks is modified while results are being merged back and
we miss some results because the iterator wont take these new items
into account *during* iteration.