update fluentd buffer configuration

Closes #45 (closed)

The current fluentd buffer conf is not limiting the max size, as a result disk pressure is possible. The new conf should improve the speed of flushing the buffer, set the max size of the buffer, and the policy for overflow (drop oldest).

Also I'm assuming that the current configuration is not actually working (probably we are relying on defaults), because to use timekey and timekey_wait we are missing the keyword time in the conf.