WL#12470: Volcano iterator semijoin

Building on WL#12074, this worklog aims to implement all forms of semijoins
in the iterator executor.

No (new) functional requirements. All queries should remain the same. Even 
performance schema results should be the same. Some warnings may end up on 
different line numbers.

No (new) non-functional requirements. Performance should not regress.

We currently have four semijoin strategies, which map reasonably well to the
iterator model:

 - First match: Extend NestedLoopIterator with a new semijoin join type,
   which works like an inner join except that it stops after the first match
   on the right-hand side. Some corner cases will need to be handled by inserting
   LIMIT 1 instead.

 - Semijoin materialization: Straightforward using MaterializeIterator.

 - Duplicate weedout (join regularly, then remove duplicates): Make a new
   iterator that reuses the logic from do_sj_dups_weedout(). Some work is
   needed for the rare cases when the join optimizer sets up non-hierarchical
   weedouts that happen to interfere with outer joins.

 - Loose scan (remove duplicates from ordered row set, then join regularly):
   Make a new RemoveDuplicatesIterator to remove duplicates in a streaming
   fashion. For loose scan involving multiple tables, we need a new combined
   nestloop + loose scan iterator to maintain the current efficiency.