defer: Local cache mutation writes

[calvincestari]

Getting local cache mutation cache writes working with operations that use @defer.

• Removes the assertionFailure and collects fields to execute if the deferred fragment is marked as fulfilled.
• Updates documentation for cache write caveat (using private DataDict initializer).
• Removes old TODO markers, test reminders, etc.

[AnthonyMDev]

LGTM!