Change JSON formatters to be async

[kichalla]

Latest JSON.Net package supports async reading and writing.
http://james.newtonking.com/archive/2017/03/21/json-net-10-0-release-1-async-performance-documentation-and-more

cc @DamianEdwards @pranavkm @rynowak

[kichalla]

The async apis are not available on the JsonSerializer type (which we use here and here) yet, so looks like we cannot move to async yet. Async support is available on JsonWriter and JsonReader but that doesn't help us here.

[kichalla]

@JamesNK Is support for exposing async api on JsonSerializer part of your future plans?

[JamesNK]

Maybe one day but not right now. It will be a lot of work

[kichalla]

Thanks @JamesNK .

Closing this issue for now as there is no action item here.