pubsub.asyncIterator should return an AsyncIterable

[hayes]

See apollographql/graphql-subscriptions#261

This is the exact same issue as described in the issue linked above.

The returned AsyncIterator also implements the AsyncIterable interface (having an [Symbol.asyncIterator] property). GraphQL subscriptions expect an async iterable, not an async iterator, so this works at run time, but the typings are incomplete. Many of the type-definitions for subscribe functions are wrong, so this issue has gone un-resolved for a long time, but technically the correct thing here is to update the return type so that it correctly indicates that the returned value is both an AsyncIterator and an AsyncIterable.