Implementation of guaranteed delivery from ForwardOutput to ForwardInput

This patch provides "At least once" transferring on out/in-forward protocol.

This pull request is re-request of https://github.com/fluent/fluentd/pull/422.
And added patches below:
 * rebase w/ v0.12 code
 * use `chunk.unique_id` as ack value
 * implicit configuration variable: extend_internal_protocol
  * assume from other configuration values
 * default ack timeout: 190secs from linux SYN_ACK defaults
 * raise error for ack timeout to retry to send

Extended protocols are to be done with version-up of this plugin, not to be done as inheritance, I think.
We cannot maintain protocol compatibility for another plugins. That is almost impossible.

Extending point is for incoming version up of this forward plugin and its new features.