[WIP] Async (2nd try)

Continued from https://github.com/yohoushi/fluent-plugin-yohoushi/pull/6

Okay, we met troubles with httpclient, so I renewed grothwforecast-client as it uses `net/http` rather than `httpclient` on the pull-req https://github.com/sonots/growthforecast-client/pull/6. I actually suspected `timeout` codes of httpclient https://github.com/nahi/httpclient/blob/master/lib/httpclient/session.rb#L876-L898. `net/http` looked taking care timeout properly using `read_nonblock` and `IO.select` https://gist.github.com/sonots/8784780

It looks working well, but let me wait and see how it works for a while.