Getting MonoTouch client builds working with latest source.

As I mention in the comments in the `AsyncServiceClient` changes, I wasn't sure why the position of the stream was attempting to be reset. I couldn't find anywhere that the stream is used again later after an error, and the sync version `ServiceClientBase` doesn't attempt to do anything similar, so I am assuming this is old legacy code that can probably be removed completely, but I left it as a #define for MT for now.