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 what this is trying to do here.  This is a terrific idea for http clients and a not very good idea for node clients.  I will explain further in a full comment (rather than just this line).