Based on NdrStubCall2 and NdrAsyncClientCall implementations.

In order to test asynchronous RPC I used dlls/rpcrt4/tests/server.c as a base,
converted int_return() and sum() to use asynchronous RPC on both client and
server sides (server.idl doesn't need any changes), and added server.acf:
interface IServer
{
    [async] int_return();
    [async] sum();
}
With this implementation these tests pass under Wine. Since widl doesn't
support asynchronous RPC it's impossible to add appropriate Wine tests.
Signed-off-by: Dmitry Timoshkov <dmitry@baikal.ru>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>