After a tail watcher transitions to `rotate_wait` state, the
`rotate_wait` timer is no longer managed by in_tail, it might cause
unexpected behaviour. e.g.)

* It's never unwatched when shutdown occurs before `rotate_wait` passed.
* Needless `rotate_wait` timers are executed when it detects more
  rotations.

This patch fixes such unexpected behaviour.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Logic inside `detach_watcher()` should be independent of
`rotate_wait`.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

The comments was added in 76f246a.
At that time, closing was done by event-loop.
Now, the situation is completely different.

Currently, there are comments about the `close_io` option in
`shutdown()` and `close()`.
So, it would be enough to remove these comments.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

`sleep 1s` is too short for `enable_stat_watcher false`.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>