[WIP] Update hub to use execute_(request|reply)
[WIP] Update hub to use execute_(request|reply)

Update hub to use `execute_request` and `execute_reply` messages for
requests and replies on the *query* socket.

TODO
----

 - Handle forwarding of `execute_request` and `execute_reply` messages
   through the *command* socket.
 - Update *publish* socket handling to publish JSON messages (not
   pickled Python objects).

wheeler-microfluidics/zmq-plugin