Support for suspendable schema execution

[mdnorman]

Along with #38, it would be great if the schema supported suspendable execute. That way, when using a server that supports coroutines (like ktor with netty), expensive queries and mutations could be suspended naturally with low overhead.

[jeggy]

I opened a pull request solving #38. I think that is solving most of the suspendable executions.

But I'm wondering why even have suspendResolver and not just make everything suspendable. The logic behind is doing it anyway. This is also how ktor does it.