The main change in this commit is changing the return type of
SubscriptionType::create_stream from
Future<Result<Stream<Result<Response>>>> to just Stream<Result<Json>>. I
also allowed the returned stream to borrow from self and context.
I think the previous implementation is not elegant enough, the `QueryBuilder::set_files_holder` function looks disgusting, so I refactored it.
By the way, the performance of parsing InputValue has been optimized, and unnecessary clones have been removed.
