bufferize fluent-plugin-yohoushi

I noticed that fluent-plugin-yohoushi should be buffered because, when yohosuhi (growthforecast) shutdowns accidently, non-buffered fluent-plugin-yohoushi will retry serveral times and consequently blocks processing messages. 

fluent-plugin-yohoushi should not block, and befferizing (BufferedOutput) plugin is the easiest way to achieve it.