defer: Deprecate legacyResponse

[calvincestari]

Closes apollographql/apollo-ios#3370.

• Deprecates legacyResponse
• Stores the cache keys which are a result of the JSON interceptor parsing

[AnthonyMDev]

This LGTM! Crazy how little code was actually needed to do this and get such a big performance optimization. We should have done this a long time ago. Thanks so much @calvincestari!

I'm not sure why you changed the indentation styling of a bunch of the code in this PR as well. I prefer the style with params on individual lines, but since we haven't adopted an official style guideline and linter rules, I'm not going to request changes for that. That is something we really should do though 😅 .

[calvincestari]

This LGTM! Crazy how little code was actually needed to do this and get such a big performance optimization. We should have done this a long time ago.

I still need to:

• add fast parsing tests for IncrementalGraphQLResponse
• clean up the failing codegen tests due to warnings about an unused parameter in the incremental interceptor
• capture performance tests of eliminating the double parsing when writing to the cache

[calvincestari]

Ideally the parseResult and parseResultFast functions in GraphQLResult would be internal rather than the public scope they have now but that would be a potentially breaking change. These (parseIncrementalResult and parseIncrementalResultFast) are scoped minimally for now but we can always loosen that if needed.