Only cache complete responses for deferred operations

[calvincestari]

This issue may break out into more but at a minimum the cache write interceptor will need to disregard partial responses and only write the record to the cache once a complete response has been received.

[calvincestari]

Closing this as the caching strategy for deferred responses has changed; we will instead be caching partial and incremental responses. That work will follow after PR apollographql/apollo-ios-dev#219.

[github-actions]

Do you have any feedback for the maintainers? Please tell us by taking a one-minute survey. Your responses will help us understand Apollo iOS usage and allow us to serve you better.