Support fluentd new worker model

[riywo]

http://www.fluentd.org/blog/fluentd-v0.14.12-has-been-released

• Plugin MUST be implemented with v0.14 API natively
  • Plugin class is a direct subclass of Fluent::Plugin::Input, Fluent::Plugin::Output, Fluent::Plugin::Filters, etc
• Plugin MUST have #multi_workers_ready? method and return true
  • The default return value of this method is true in filter/buffer/parser/formatter/storage plugins
  • But it's false in input/output plugins
  • 3rd party plugins SHOULD re-implemnet #multi_workers_ready? to return true to support multi process workers
• Plugin MUST NOT use raw sockets to listen network ports
  • It makes Errno::ADDRINUSE errors with multi process worker configurations
  • Use server plugin helper to create networks servers to listen network port via the supervisor process

[riywo]

#112