Make batch delegate send key arrays to loadMany

[gmac]

It's not uncommon for a model to posses an array of keys to be stitched from another service, for example:

type Post {
  networkIds: [ID!]!
}

In these cases, the array of keys are delegated through DataLoader.loadMany so that the results return an array of mapped models. This PR adds this behavior. While at it, I've added eager returns for skipping delegation entirely when a key or key set is empty.

TODO:

• 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

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 6.0.16-alpha-7c55fbe4.0

Quickly update your package.json by running:

npx match-version @graphql-tools 6.0.16-alpha-7c55fbe4.0