[frontend] pagure-events: send keep-alive tcp packets
    
    After e.g. Pagure restart (or dunno what were the conditions), we
    started to see only:
    
        ...
        [2019-04-19 13:50:06,267][ DEBUG]: Polling...
        [2019-04-19 13:50:16,279][ DEBUG]: Polling...
        ...
    
    in our log, even though pagure _was_ sending the messages.  So
    setup some defaults for tcp keep-live packets (we'll move to
    fedora-messaging (amqp) from fedmsg (zmq) anyways, so this is
    rather temporary fix).