Create the tasks using threadpool mapped functions

These functions should/could be using the threadpool:

• engine_make_hydroloop_tasks()
• engine_make_self_gravity_tasks()
• engine_count_and_link_tasks()
• cell_set_super()
• engine_make_hierarchical_tasks()
• engine_make_extra_hydroloop_tasks()
• engine_check_sort_tasks() (removed in 145b4377)
• scheduler_ranktasks() (doesn't parallelize)