Various memory optimizations

[wravery]

This should fix #138, #140. Overall the parsing speed and memory usage is much improved.

[barbieri]

Any reason you didn't pick the following commits?

• 8579b1e
• 62ce891

[wravery]

Any reason you didn't pick the following commits?

• 8579b1e
• 62ce891

I baked that into ModifiedResult<>::resolve instead. Since FieldResult<> is part of the signature for all of the user code I'd rather keep it simpler and easier to understand and encapsulate the complexity in the internals of GraphQLService.h.

Switching to std::async::deferred is there for the converters when it holds the std::future<> alternative, otherwise it uses the approach you had where it creates an immediately-fulfilled std::promise<> and returns that future. It still uses params.launch when peeling off the list modifiers and when visiting fields in SelectionSetVisitor because those are potentially parallelizable.