[ASDataController] Remove asyncDataFetching Option, Cleanup

[Adlai-Holler]

I was doing a deep-dive into this code in preparation for some work, and I saw a bunch of possible improvements. So here they are.

• Remove asyncDataFetching logic, deprecating the associated methods. We no longer call the locking methods because we now follow the UIKit synchronization paradigm – submit your edits on the main queue, and we'll query you back immediately on the main queue.
• Add assertions to data controller about which queue we're on
• Make data controller use _editingTransactionQueue even when doing synchronous reload (so that those assertions pass).
• Other small tweaks/optimizations (avoiding needless allocations/method calls)

I've tested this thoroughly by hand.

[Adlai-Holler]

@maicki @appleguy Review?

[Adlai-Holler]

Those last two commits were going a little crazy, but here's the evidence for my comment about [NSIndexPath indexPathForItem:inSection:]:

[appleguy]

Hmm, is this guaranteed to properly initialize the .origin values? Or are they left uninitialized?

[Adlai-Holler]

Yep! C++ will initialize all struct members to 0 by default so {} => CGRectZero etc.

[appleguy]

@Adlai-Holler thank you for including the categories and deprecation flags to maintain compatibility. I reviewed this in quite heavy detail, but I am also partially trusting that there are not material changes subtly within some of the code blocks that were removed from the access data source with block sections.

[appleguy]

@Adlai-Holler the detail is quite interesting on using Singleton objects for index paths only when initialized with the to argument method, thanks for including that.

Can you please add a comment where are you added the fork code path, because it is far from obvious why it would be worth the complexity to do so?