Expose parent on CacheKV (#352)

## Describe your changes and provide context
For EVM getProof endpoint, we'd need to call functions on the underlying
iavl store, so we'd need a way to expose the parent store from a cachekv

## Testing performed to validate your change

store/cachekv/store.go

@@ -359,3 +359,7 @@ func (store *Store) isDeleted(key string) bool {
 	_, ok := store.deleted.Load(key)
 	return ok
 }
+
+func (store *Store) GetParent() types.KVStore {
+	return store.parent
+}

store/cachekv/store_test.go

@@ -62,6 +62,9 @@ func TestCacheKVStore(t *testing.T) {
 	st.Write()
 	require.Empty(t, st.Get(keyFmt(1)), "Expected `key1` to be empty")
 	require.Empty(t, mem.Get(keyFmt(1)), "Expected `key1` to be empty")
+
+	// GetParent returns parent store
+	require.NotNil(t, st.GetParent())
 }
 
 func TestCacheKVStoreNoNilSet(t *testing.T) {