'Invoked when we create a new session, with a blank Document that
hasn\'t been filled in yet.
May return a Future which will delay session creation until the Future completes.'

@gen.coroutine
def on_session_created(self, session_context):

    for h in self._handlers:
        result = h.on_session_created(session_context)
        (yield yield_for_all_futures(result))
    raise gen.Return(None)
