Introduce OptimisticNormalizedCache and API to write/rollback

[sav007]

Introduce OptimisticNormalizedCache to be always used as first layer in a chain
Introduce ApolloStore API to write/rollback optimistic updates
Some small refactoring related to RecordFieldJsonAdapter that required only for SQL cache

Part of #583

[BenSchwab]

Maybe I'm missing something, but does the following work:

Current key value (not in optimisitc cache): A -> 0
Optimistic update 1 A -> 1
Optimistic update 2 A -> 2
rollback optimistic update 2:
expect A -> 1, but it seems that rolling back 2 would remove the A from the optimistic cache, forgetting about update 1, which is still pending and valid.

Granted, the whole concept of racing contradicting mutations seems problematic to begin with.

[sav007]

@BenSchwab yes that is correct, updates with version 2 will override updates with version 1. And when we rollback the version 2 it will rollback all. This case will happen only when second query was faster than the first one.

In order to support this case I guess we need to keep the whole history of changes for all records in mem cache. I think for now we can keep this simple version of implementation, and I will think how we can solve this case.

[BenSchwab]

Cool, well this looks neat.
Is the next part exposing this as part of the ApolloMutationCall for convenience?

[sav007]

@BenSchwab Yeah, it will be almost similar to JS: ApolloMutationCall.optimisticUpdates(...)