Abrandoned/config zmq

Moving the values that were hardcoded into the zmq broker/connector into environment variables with "reasonable" defaults (the defaults basically maintain the behavior before this PR)

`PB_ZMQ_CLIENT_HOST_ALIVE_CHECK_INTERVAL` - how many seconds between health checks
`PB_ZMQ_CLIENT_SERVER_LOOKUP_ATTEMPTS` - how many times to lookup the server in the service directory list
`PB_ZMQ_SERVER_BACKEND_POLL_WEIGHT` - in the broker this is how many backend reads will be executed if the backend socket is readable while processing the backend responses (defaults to 1)
`PB_ZMQ_SERVER_FRONTEND_POLL_WEIGHT` - in the broker this is how many frontend reads will be executed if the frontend socket is readable while processing the frontend requests (defaults to 1)

@liveh2o @localshred @quixoten 
Any reason not to use `+=` here?