ElasticSearch output plugin to support multiple hosts and enhance stability 

ElasticSearch output plugin to support multiple hosts and enhance stability
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1. Make elasticSearch output plugin to support mutiple hosts and round robin balance, which will help a lot to balance the resource and performance of whole elasticsearch cluster.
2. Improve stability by handling exception and retry with left servers while flushing bulk, which will reduce the possibility of data loss largely.
I appreciate that you do not object to some event duplication. Would all of our other users feel as you do?  I can't answer that for them.  This particular detail is a sticking point which will prevent the patch from being merged until it can be made to only retry failures and to properly log true failed lines for post-mortem analysis.