Batch requests

[nightscape]

Ok, this is ugly as hell, but maybe a starting point for discussion.
I've changed the logic of HttpDirectiveProvider to return a RequestDescription <: Deferred[Json] and created a RequestResolver <: DeferredResolver[GatewayContext] which performs the actual logic.
In principle one could now go ahead and transform a Vector[RequestDescription] into batch requests, but there's too many things that feel just awfully wrong:

1. RequestDescription needs to store the entire Context[GatewayContext, _] so that the RequestResolver can execute its logic. It would be much nicer if it would suffice to have a GatewayContext which would be the Ctx of RequestResolver anyway, but Context[GatewayContext, _] is needed for fillPlaceholders.
2. I don't know if it's ok to just replace the schemaInfo.deferredResolver. It seems that it's an empty resolver by default.
3. It would be much nicer to use the high-level Fetcher API. The point where I got stuck with this is that a Fetcher needs a HasId[Res, Id], so Id can't be something complicated like my RequestDescription. But if I make Id something simple like a Json object, I wouldn't know how to get all the other required things into the Fetcher.

If you have any ideas how to make this nicer I'd be very thankful 😃