Perform super-graceful shutdown of the socket on success.

This should guard against a RST being sent on success.  It's possible
that the remote has sent data (ie: M_EOB) we would recv() after we
commit to ending the session.  If the script terminates while there
is data to be read, this would result in sending a RST.

To prevent this, we call shutdown(sock, SHUT_WR) via setting is_writeable
to false (because that's how we roll), then recv() all data until the
remote closes the session, or the timeout passes.