Optimistic updates with fake IDs

[AlecAivazis]

When a user is submiting an optimistic update, their payload won't know the correct id to provide for a new element that they want to add to some list. This might create some weirdness if the optimisic id is used as the input for a mutation. Ideally, the cache would be able to recognize that the id being used as a mutation input and block the requests until the original mutation resolves and provides a correct id.

This feature could be opted into by asking for the id of the mutation payload explicitly. There would have to be a way for the generators to identify that id wasn't asked for by a transformation.

[AlecAivazis]

Here's where I'm at right now with the api:

mutation CreateItem {
  createItem {
    item { 
      id @optimisticKey
      text

      ...All_Items_insert
    }
  }
}