returning handles

• when the first handle is returned, it gets added to the recently_freed list. Because this is the first item on that list, we check the time.
• now we add more handles to the list. we check the time after N handles are returned and update the timestamp.
• Once we have added H handles, we decide the recently_freed list has enough handles. We then start using the overflow_list to hold returned handles.
• as with the recently_freed list, we record the time that this handle was added, updating the timestamp after every N additions. We also check how old the recently_freed list is.
• at some point in time, the whole recently_freed list is ready to be returned to the free_list. The recently_freed list is merged into the free_list, the overflow_list becomes the recently_freed list and the overflow_list is empty.