WIP: Result reducers

[helfer]

TODO:

• call during MUTATION_RESULT
• action contains operationName
• call during MUTATION_INIT with optimistic result
• call during QUERY_RESULT
• call during SUBSCRIPTION_RESULT (-> no such action. Fix when working out subscription API)
• write tests for all of the above
• add a test that calls multiple query reducers.
• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change
• Add your name and email to the AUTHORS file (optional)
• If this was a change that affects the external API, update the docs and post a link to the PR in the discussion

Things to note:

1. The reducer will run on every result, including the result of its own query.
2. The reducer (right now) does not run on subscription results (pending subscription API change), and it does not run on query results that didn't hit the server (i.e. QUERY_RESULT_CLIENT). It also doesn't run when errors are returned. We may need to update this in the future.
3. Reducers run serially. A reducer gets the store with modifications from previous reducers already applied.
4. Reducers run after writing the result to store, and after result behaviors.

This feature could use a few more tests to make sure the interactions with other features (eg. optimistic updates) is solid.

[helfer]

@stubailo can you give this a quick look?

[helfer]

I'm going to merge this now, we can make changes later if necessary.