Mutate update function DataProxy/ApolloCache changes

[hwillson]

This PR gives the mutate update function direct access to the cache, instead of using a scaled down DataProxy. While the DataProxy approach helped in the past as a way to batch cache updates, it is no longer necessary. We are only ever passing a cache instance into update, which means limiting the type to be a DataProxy gives us no additional advantages. The only other DataProxy implementor in the AC codebase is ApolloClient, which is never passed to the update function. Also, if we keep using DataProxy in the mutate update function, then every method we want to expose from the cache through DataProxy has to be also added to ApolloClient. This doesn't always make sense to do, especially when considering cache functions like identify (e.g. ApolloClient.identify() seems off).

This PR also exposes optional identify and modify API elements in ApolloCache. Since update now has access to the full ApolloCache instance passed in, identify and modify can be used in the update function. Note that the ApolloCache implementations of identify and modify do nothing of value; they should be overridden by ApolloCache subclasses that are interested in using them, like InMemoryCache does. By including them in ApolloCache as basically no-op methods, we're avoiding forcing alternative cache implementations that subclass ApolloCache to have to support them.

[benjamn]

Thanks for going the extra mile with the DataProxy elimination and thoughtful reorganizations of types!