Use LinkedHashMap for deterministic iterations

Test `testKeyedMapStateUpgrade` depends on `put` in `HeapMapState.java` and `deserialize` in `MapSerializer.java`. It has this following test assertion `assertEquals((Integer) 1, actual.getKey());` that asserts the `getKey()` value of `actual`. However the `HashMap` in `put` and `deserialize` does not guarantee any specific order of entries. Therefore, the assertion will fail if the order is different.

This PR proposes to use `LinkedHashMap` for a deterministic order. Please let me know if you want to discuss the changes more.