Add option to disable default persistent TCP connections to ES

This commit adds new option 'http_disable_keepalives'  to ElasticSearch plugin.
Dealing with persistent TCP connection is only useful with single ES server, but becomes a headache when you have to implement some fallback functionality. We are planning to use HTTP proxy server between heka ES plugin and ES cluster and to send HTTP requests on different servers from ES cluster.

I'm not developer at all and beginner in Go, so just copied existing option 'http_timeout' and pasted it under new name. I'll be glad if this patch will be useful for anyone.