Optimize isValidating status

[anothertempore]

This PR is set the initial isValidating to be true if there is a request will be sent on mount.

Currently, if we will fetching on mount, the isValidating will change like false -> true -> false. If we using isValidating to indicate a loading status, which is a bit weird.

I think it could be better to set the initial isValidating to true if we will start fetching. So it is true -> false.

If there is anything wrong, please correct me. 🙈 Thanks!

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[shuding]

Thanks! I don't like the false -> true -> false state change either but my only concern is that this might cause the hydration mismatch. But I'm almost sure there's a way to determine and avoid that. I'll review and think about this again carefully.

[chybisov]

I don't like the false -> true -> false either and hope this PR will be merged. @shuding did you have a chance to think about it? Thank you!

[LRNZ09]

This would be very appreciated. It's odd to have the initial loading state set to false.

[shuding]

Hi @chybisov @LRNZ09 @Kexin-Li! I think we can have this merged and release a beta version, once the above suggestion is merged 🙏

Thanks for your effort @Kexin-Li @huozhi! And I need to test it a bit in a large application before releasing a stable version, because this change is quite critical. But once landed, this is gonna help the performance a lot!

Also, we need to update the docs when we release it: https://swr.vercel.app/advanced/performance

[shuding]

Thanks!

[shuding]

Released as 0.4.3-beta.2 🎉