fix(middleware/persist): Merge storage value with the latest store state

[zerofirework]

When using persist middleware, it's possible to mutate the store state in an useEffect callback before hydration creates stateFromStorage. It should merge the storage value with the latest store state instead of the config result since it can be out of date.

To reproduce:

1. create a storage whose getItem returns the value after a 2sec timeout. The storage is responsible for persisting a partial state. (property a)
2. add a useEffect to update the state right after render. It adds another property to the store. (property b)

Expected: When the storage value is fetched, it merges storage value with the latest store value.
Actual: When the storage value is fetched, it merges storage value with the initial store value.

Error demo: https://codesandbox.io/s/react-forked-oo16v5?file=/src/App.js
Fixed demo: https://codesandbox.io/s/react-forked-hydp10?file=/src/App.js

[codesandbox-ci]

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[AnatoleLucet]

Thanks for the PR, but unfortunately this change breaks quite a few tests

[AnatoleLucet]

Looks good, thanks!

[dai-shi]

Thanks for your contribution!