This new callback simply stores (without calling any lower-level
callback) the last result; coupled with the filtering callback, this
ensures that it has the 'best' response after all have been received.

The result can then be retrieved via the GetResponse method.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>