Added REQ/REP pattern available to ZMQ input

The REQ/REP (request/reply) pattern is essentially the same as REQ/ROUTER (request/router) from the REQ's perspective. There are use cases when you may want to swap out REP for ROUTER, so this addition is to use REQ on the logstash side: see http://zguide.zeromq.org/page:all#The-Load-Balancing-Pattern. This is alternative to PUSH/PULL in which the first PULL connections may grab too many messages and not be truly load balanced.