ID handling issues

1. https://gitlab.com/gitlab-org/fluent-plugin-redis-slowlog/-/blob/dd28a5e3608bf0bf94870f1c92a51fdb91571f18/lib/fluent/plugin/in_redis_slowlog.rb#L82 means it will always ignore the oldest slowlog entry because it has id 0, and current_log_id is 0 at startup.
2. When redis itself restarts the slowlog will be emptied and id's start from 0 again,
3. Conversely, when fluentd is restarted current_log_id starts from 0 again so all current slowlogs will be re-emitted.

Perhaps we should use the timestamp of the slowlog entries instead of just the ID, and store the last-seen timestamp (in the monitored redis, for simplicity) to survive fluentd restarts.

/cc @reprazent WDYT?