Implement self ref fk ordering

[ozer550]

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WIP
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TODO

• Have tests been written for the new code?
• Has documentation been written/updated?
• New dependencies (if any) added to requirements file

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[bjester]

So from our discussion on slack and seeing this, I think some of the behaviors have been taken very literally, which is my fault for not making that clearer.

if _self_ref_fk is None and the model is self-referential, the order field should be 0
otherwise, it should query for the parent's order (the Store._self_ref_order for id=_self_ref_fk)

We might need to query the database for this, but I think it's worth thinking through how and when this should occur. This satisfies the criteria of the behavior, but the engineering choices are up to you. The behaviors are not meant to be taken literally that this functionality should explicitly be placed here.

I say this for a couple reasons:

1. Look at how we manage database calls-- in StoreLookup we do bulk queries for the store records, and how StoreUpdate actually does none. There's room for improvement in managing these queries.
2. The circumstances may not always work with the pipeline because it buffers records. On this note, I'm thinking in regards to this test case test_handle_store_create__self_ref_parent_not_in_store and whether that is an acceptable outcome.

More on (2): say we have our buffer batch size set to 2. In the first batch, we have a parent and a child. Since the batch size is two, the parent won't be saved yet when the child is processed, so this function should result in None. Then lets say batch #2 has two more children, of the first child (grandchildren). The function should find the store, but the store would have a None order, leading to None for the grandchildren. Thus we have a parent-child relationship which could get out of order because of None.