bugfixes and updates to gen=py.tornado

1. Use coroutines and futures for keeping track of transport errors.
2. Add io_loop keyword to 'TTornadoStreamTransport',  'TTornadoServer' constructors.
3. Raise exception for transport open in caller context.
4. Don't wait for writes to flush on oneway methods, that's a bad
   practice when doing concurrent writes.
   (http://www.tornadoweb.org/en/stable/iostream.html#tornado.iostream.BaseIOStream.write)