r2100: rework the dcerpc client side library so that it is async. We now

r2100: rework the dcerpc client side library so that it is async. We now
generate a separate *_send() async function for every RPC call, and
there is a single dcerpc_ndr_request_recv() call that processes the
receive side of any rpc call. The caller can use
dcerpc_event_context() to get a pointer to the event context for the
pipe so that events can be waited for asynchronously.

The only part that remains synchronous is the initial bind
calls. These could also be made async if necessary, although I suspect
most applications won't need them to be.
(This used to be commit f5d004d8eb8c76c03342cace1976b27266cfa1f0)