Feat(GraphQL): Make XID node referencing invariant of order in which XIDs are referenced in Mutation Rewriting

[vmrajas]

Currently, in rewriteObject function, the fields inside given Add / Update Mutations are sorted before iterating on them and recursively calling rewriteObject function. This behaviour ensures that the fields are considered in the same order in rewriteObject and existenceQueries function.

The drawback this as is follows,

1. If a there is a node which is created and referenced in the same mutation, depending on the order in which the nodes (creation and reference) get visited, an error may be thrown.

This issue has been present even before the refactoring. This PR fixes this.

Testing:
Added unit test in .yaml

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This change is 

[pawanrawal]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @vmrajas)