fix and add linger_timeout option to in_forward

I've noticed that @timeout paramter at https://github.com/fluent/fluentd/blob/d5b4dc16e061d4f0faf6b63a6107220137653b06/lib/fluent/plugin/in_forward.rb#L151 was always nil. 

MEMO: Since linger_timeout is 0, in_forward returns RST instead of FIN. This is effective to clear TIME_WAIT sockets at src. See http://stackoverflow.com/questions/3757289/tcp-option-so-linger-zero-when-its-required