chore: optimize the magic number to Option and extract common code

[1996fanrui]

Changes:

• Refactor the type of rolling.State#next_date from usize to Option<usize>
  • rolling.State#next_date is 0 when rotation is Rotation::Never.
  • Using Option compared to magic number can intuitively tell developer that next_date is not effective.
• Move date.unix_timestamp() as usize inside of next_date
  • All callers of Rotation#next_date convert the data to timestamp, we can convert it inside of Rotation#next_date
  • Also, Rotation#next_date needs to rename to Rotation#next_date_timestamp

[1996fanrui]

Hey @tisonkun , would you mind taking a look this minor refactor in your free time? thank you very much.

[tisonkun]

LGTM. One minor comment inline.

[tisonkun]

Since we touch a few times the rolling file implementation, I'll appreciate it if you can try to port some tests in https://github.com/tokio-rs/tracing/blob/master/tracing-appender/src/rolling.rs so that we can guarantee the rolling behavior kept during changes.

[tisonkun]

Thanks for your contribution!

[1996fanrui]

Thanks for the quick review and merge!

Ahhh, I addressed your comment and I'm trying to add some teets on my Mac.

I will create a new issue to add test later due to it's already merged.

[tisonkun]

@1996fanrui OK. I may be too hurry to merge a PR :D