fix: prevent race condition in KeyValueStore.getAutoSavedValue() (#2193)

This was painful to discover the hard way :(

Code to test this:


```js
const store = await Actor.openKeyValueStore("test", { forceCloud: true });

const key = "something";
const res = await Promise.all([
  store.getAutoSavedValue(key, { foo: [] }),
  new Promise((r) => setTimeout(r, Math.random() * 1000)).then(() =>
    store.getAutoSavedValue(key, { foo: [] })
  ),
]);
res[0].foo.push("test");
console.log(res);
```

Run this a few times and at some point you will get two different values
in the console.log. (Note that it's using `forceCloud` to make
`this.GetValue()` run for a little longer.)

packages/core/src/storages/key_value_store.ts

@@ -227,6 +227,15 @@ export class KeyValueStore {
         }
 
         const value = await this.getValue<T>(key, defaultValue);
+
+        // The await above could have run in parallel with another call to this function. If the other call finished more quickly,
+        // the value will in cache at this point, and returning the new fetched value would introduce two different instances of
+        // the auto-saved object, and only the latter one would be persisted.
+        // Therefore we re-check the cache here, and if such race condition happened, we drop the fetched value and return the cached one.
+        if (this.cache.has(key)) {
+            return this.cache.get(key) as T;
+        }
+
         this.cache.set(key, value!);
         this.ensurePersistStateEvent();