feat: better autorenewal

[k3-cat]

Background

Currently tokens only get renewed when they are expiring within next 24 hours. This requires users to login at least once per day to keep them loged in -- which can be really inconvenient.

Changes & Reasons

This PR allow tokens to be renewed when their remaining lifetime is less than a third of the configured value. The threshold a third is come from those common certificate autorenewal configuretions -- for example, the default config of certbot.

[trofen]

I agree that the current automatic token update doesn't work as well as expected, but this pr is not the best solution either. I would suggest @lejianwen to merge it, but consider a better solution in the future

[k3-cat]

Absolutely not. It's just intended to make a little improvement while minimize impact on existing config/db/code and keep performance overhead low (which is why I use raw milliseconds, and hence integer division).

Still, I doubt there would be an ideal solution for rustdesk - since it only uses a single access token and lack of refreshing mechanisms - but I do have some ideas about it:

• Use a layer of in-memory cache (radius would work, but probably overkilled, something like go-cache would suffice) to temporarily hold checked tokens
• When a user accesses the API, store a timestamp to the cache - the cache is key-value based; "key" is the token, and "value" is the timestamp
• Having a task that periodically read the record from the cache, calculate a new expiration time, and write to the database
• When the cache expires (either due to timer or lur), immediately execute the previous flow

This is a pretty trivial approach... but...
Pros: easy to implement, tokens can be rolled smoothly
Cons: cache hell, need to manually manage cache state, too many extra components