use session.send throughout zmq code

[minrk]

For compatibility between the interactive and parallel codes, as well as enforcing valid and uniform messages throughout, all message sends should be done via session.send(), and methods like socket.send_json should not ever be called anywhere outside the Session object itself.

grep 'send_json' IPython/zmq should probably return zero results outside of IPython/zmq/session.py

[minrk]

This was closed by #250