MWL#90: Subqueries: Inside-out execution for non-semijoin materialized subqueries that are AND-parts of the WHERE
- Change "SUBQUERY#n" to "<subquery{n}>" in EXPLAIN output. We need to it to be
  lowercase so that EXPLAIN results do not differ in case between systems with
  case-sensitive and case-insensitive filesystems.
- Remove garbage comments, add better comments.