Allow field getters to return a std::shared_ptr<const response::Value>

[wravery]

In cases where copying elements between service::ResolverResult documents and converting to response::Value adds a lot of overhead, field getters can now hold on to a pre-converted std::shared_ptr<const response::Value> and return that. The ModifiedResult and FieldResult will prefer that over a value of T or std::future<T>, and response::Value can now perform copy-on-write from a std::shared_ptr<const response::Value>, so it doesn't need to repeat any conversion or copying to keep returning the same shared_ptr.