qbt runs backtests using multiple processes and zeromq messaging for communication and coordination. 

Backtest is the primary process. It maintains both server and client sockets:
zmq sockets for internal processing::

    - data sink,        ZMQ.REQ. Port = port_start + 1
        - backtest will connect to socket, and then spawn one process per datasource, passing the data sink url as a startup arg. Each
          datasource process will bind to the socket, and start processing 
        - backtest is responsible for merging the data events from all sources into a serialized stream and relaying it to the 
          aggregators, merging agg results, and transmitting consolidated stream to event feed.
    - agg source,       ZMQ.PUSH. Port = port_start + 2
    - agg sink,         ZMQ.PULL. Port = port_start + 3
    - control source,   ZMQ.PUB.  Port = port_start + 4
        - all child processes must subscribe to this socket. Control commands:
            - START -- begin processing
            - TIME  -- current simulated time in backtest
            - KILL  -- exit immediately
            
zmq sockets for backtest clients:
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    - orders sink,      ZMQ.RESP. Port = port_start + 5
        - backtest will connect (can you bind?) to this socket and await orders from the client. Order data will be processed against the streaming datafeed.
    - event feed,       ZMQ.RESP. Port = port_start + 6
        - backtest will bind to this socket and respond to requests from client for more data. Response data will be the queue of events that
          transpired since the last request.