Allow resolver fn to be async

[maclockard]

Looking at some possible downstream consumers, these seems fine and not breaking since they can receive an async subscription function:

• https://github.com/graphql/graphql-js/blob/master/src/subscription/subscribe.d.ts#L42-L51
• https://github.com/apollographql/subscriptions-transport-ws/blob/master/src/server.ts#L69-L78

fixes #213

[XavierLeTohic]

Any news on this?

[AadamZ5]

@hwillson Can you please glance at this? You're the most recent active contributor of this project. This changes similar files from a PR you've recently merged #229.

Many dependent libraries would benefit from this contribution. Currently, many developers have to employ ugly workarounds to get around this limitation. The lack of async operability is frankly extremely inconvenient in a handful of scenarios.

[hwillson]

Sorry this took so long @maclockard - we'll aim to have this out in 3.0 (which will be released soon). Thanks!