opt: use buffered writes lock elision with SFU subquery

When using buffered writes, the optimizer is sometimes able to elide the locking on Puts or Dels when the initial scan of the mutation was locked. For example:

```sql
SET CLUSTER SETTING kv.transaction.write_buffering.enabled = on;
CREATE TABLE ab (a INT PRIMARY KEY, b INT);
INSERT INTO ab VALUES (6, 6), (7, 7);
\set auto_trace=kv
UPDATE ab SET b = 16 WHERE a = 6;
-- Put /Table/104/1/6/0 -> /TUPLE/2:2:Int/16
```

It would be nice if the optimizer could also elide locking when a SFU subquery is used, for example:

```sql
UPDATE ab SET b = 17 WHERE a IN (SELECT a FROM ab WHERE a = 7 FOR UPDATE);
-- Put (locking) /Table/104/1/7/0 -> /TUPLE/2:2:Int/17
```

Jira issue: CRDB-50615