Only rely on native GWT supplied interfaces like the GWT version of
AsyncCallback and StatusCodeException. This allows us to later think
about dropping gwtjsonrpc as a project dependency once everything has
been converted and no more JSON-RPC 2.0 interfaces exist.
Given the size of this change, its better to do it before too many
more REST style interfaces are written and depend upon gwtjsonrpc.
Change-Id: I7a9f8b73c3612bf7a55a7ec5f82165f8a5cd7107
06bfb9030a