re-implement support for @defer directive

[lilianammmatos]

I created this PR to get the @defer changes up-to-date with master because we'd like to start using this feature internally. I'm opening this PR in-case it's useful to any one else!

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This PR reimplements the proposed addition of @defer support to Apollo Server originally proposed by @clarencenpy. Full details of the proposed spec can be found here.

TODO:

• Update CHANGELOG.md (include reference to issue & this PR)
• 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

Via the original pull request:

Summary:

• This PR uses a modified version execute.js from graphql.js. Functions that are untouched have been removed, and exported from graphql directly.
• execute() returns ExecutionResult | DeferredExecutionResult, the latter being if @defer has been applied to any fields.
• DeferredExecutionResult wraps an ExecutionResult containing the initial response, and an AsyncIterable.

Testing:

• I have extended the star wars test suite to add tests for @defer, while making sure that all the other tests are still passing. Tests are in __tests__/starWarsDefer-test.ts

[apollo-cla]

@lilianammmatos: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[Xample]

What is the status of this PR ? Is or will it be any official support from the Apollo team?

[jamiter]

I added some feedback but removed it because the actual code was added in #1287

Please disregard this.

[lilianammmatos]

Hi everyone! I, along with my colleague @robrichard, have been working on adding support for Relay-compatible @defer and @stream directives to Graphql.js in hopes of these directives making their way to the official GraphQL spec! We've opened a pull request already for these updates: graphql/graphql-js#2319

I'll be closing this pull request as this approach is not compatible with Relay's @defer implementation.