SQL Engine assumes all the pointers are valid during the computation period while it is not always true

**Bug Description**
SQL Engine assumes all the pointers are valid during the computation period,
while it is not true in the storage engine.

For example,
for `*_cate` (see issue #2931), sql engine assumes that the `StringRef`s are valid during the whole computation, while in the `Iterator` implementation (see pr #2939), the `value` will become invalid/freed after the next call of `Next`.
- `FullTableIterator`: fixed in #2939, guarantee the `value` is valid in the whole life cycle of `Iterator`
- `RemoteWindowIterator`: as `sliding window` copies the rows, we didn't change the current behaviour for now
- For local `Iterator`, we haven't fixed it for now.

**Expected Behavior**
We have to work out a good solution between SQL engine and storage engine to make them have the same assumption.

**Relation Case**

**Steps to Reproduce**

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