Use new context apis to track connection portal.

Responder-side XioConnection objects now aggregate their
associated xio_session, XioPortal/xio_context, and xio_connection.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Call xio_send_message from a well-defined portal event loop.

XioPortal maintains a queue of messages, protected by a spinlock.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>