Go Zeromq Zmq4 Versions Save

    zmq4: add multi-part Send API
    
    benchstat between Send and SendMulti:
    name               old time/op  new time/op  delta
    PubSubSendMulti-8   26.9s ± 0%    7.2s ± 0%   ~     (p=1.000 n=1+1)