User OrderedDict to retain FIFO order of task processing

[Lyalpha]

The switch to a set to handle the tasks in TaskQueue means we don't get duplicate tasks, at the expense of my_set.pop() returning arbitrary entries.

OrderedDict has the best of both worlds:

• keys remain unique, so we can _put a task using my_dict['task_info'] = None and it will replace the same entry, and retain the same index position in the dict.
• there is the my_dict.popitem(last=False) to _get a task in a FIFO sense.

Easy to add.