I needed this change because I found a bug:
1. Create a block with children in a content library
2. Delete that content library
3. Create a new identical block with children in a new content library.
4. If the OLX is identical to the original block, this new block will not load in the LMS.
The reason for the bug is that the .children field contains usage keys (which encode the library, for example), but the values were being stored in BlockstoreFieldData which caches really aggressively and caches based on the hash of the OLX. Since the OLX is identical, it assumes the .children values should be identical as well.
The fix was to move children to a children-specific field data store, and only store the part of the child data that is encoded by the OLX (the <xblock-include> data) in BlockstoreFieldData. This is a better match for the way the caching works and cleaned up a hacky part of the runtime (at least it's slightly less hacky now).
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