Add asynchronous connection primitives.

We're of course missing some core non-blocking functions still, as I can see you already noted in a comment.  You may want to check the stub code in particular, and feel free to rename stuff like ?startonly and the new constructors.  Sorry if I'm triggering many releases.  It has been nice to have the new version quickly available.  I think this should complete the async interface, or are we missing something?  I'll send a separate request for set_single_row_mode in a moment.