Add retry logic to zeromq filter

This was stripped out from pull request https://github.com/elasticsearch/logstash/pull/976 after 1.4.0 moved zeromq to logstash-contrib.

If the remote zeromq endpoint goes away, logstash never recovers or tries to reconnect. Also, there is no timeout. This code adds two parameters "retries" and "timeout", and implements them.

To give credit where due, the poller code was borrowed from https://github.com/jondot/roundtrip/blob/feature/0mq_endpoint/examples/zeromq_client.rb