ElementT - The type of each individual element to be batched.
ElementResultT - The type of the result for each individual element.
@BetaApi(value="The surface for batching is not stable yet and may change in the future.")
@InternalApi(value="For google-cloud-java client use only.")
public abstract class BatchEntry<ElementT,ElementResultT>
This class contains the element and its corresponding unresolved future, which would be resolved
when batch is successful or failed.