It's better that `TailWatcher#on_rotate` calls this `start_watchers` and `stop_watchers` instead of using RotationRequest. Because:

* As I mentioned at https://github.com/fluent/fluentd/pull/277/files#r10896152 , some users need to set large `@rotation_wait` to not miss logs.
  * For example, if their HTTP server takes 30 seconds to process a request at most, they need to set `rotation_wait 30` to not miss any logs.
* Current implementation doesn't start watching new file for `@rotation_wait` seconds. It makes the total throughput slow.
  * For example, if they have `rotate_wait 30` setting, tail plugin stops for 30 seconds every time the log file rotates.