Output plugin behavior should be configurable for expiration of retries

Currently (in working branch right now), Fluentd output plugin will **clear** buffer queue if plugin doesn't success to `#write` queued chunks in configured times or until configured timeout.
But there should be 2 options, and it should be configurable by users.
- **clear** buffer queue
- **shutdown** whole Fluentd process

The latter option requires to add system config parameter and hack for supervisor (ServerEngine).
This fix will be done as `v0.14.x` minor updates.