new feature for stale time

[lxjwlt]

SWR would't send requests of same key anymore but return the cache of it within stale time

when stale timer expires, SWR would resend request of current page on mounted or when we mutate it manually

is this feature needed and feasible？

[shuding]

Yeah this will be a useful feature, but needs more discussions on the detailed behavior. So feel free to open a new issue 👍