Make sure mutate invalidates concurrent requests

[shuding]

X-ref: https://github.com/nandorojo/swr-react-native/issues/18

We need to make sure that mutate() invalidates concurrent requests and deduplicates them. It’ll be similar to this test case, but without a target value (only revalidates): https://github.com/vercel/swr/blob/d45b05dd3c2fc2e0627bd916f1a346fc3e1b1e5d/test/use-swr-local-mutation.test.tsx#L361-L384

[shuding]

It turns to be introduced in #1498 which I think should be the correct behavior, that mutate() will not be deduplicated by default because it was triggered explicitly by the user. So if calling two mutate()s in a row:

mutate()
mutate()

It's expected to have 2 revalidation requests started.

However requests fired with useSWR(key, fetcher) should be deduplicated, if we do mutate(). This is working as expected.

I think the solution here is to have a way to manually revalidate with deduplication enabled. We'll have to come up with a new option for that...