Fix several failure cases.

1. handle xio_send_msg failure (XioMsg and XioConnection refcnts)
2. handle xio_release_msg failure (report only)
3. handle enqueue_for_send after shutdown
4. don't redup (xio test case behavior) before/after disconnect
5. call messenger::shutdown before exit in xio_client.cc

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