* feat(optimizer): MCOL-5250 rewrite queries with DISTINCT
... as aggregated queries.
So query
```
SELECT DISTINCT <cols list>
FROM <from list>
WHERE <where clause>
HAVING <having clause>
ORDER BY <orderby list>
LIMIT <limit>
```
will become
```
SELECT *
FROM
(
SELECT <cols list>
FROM <from list>
WHERE <where clause>
HAVING <having clause>
) a
GROUP BY 1,2,3,...,N
ORDER BY <orderby list>
LIMIT limit
```
* move ORDER BY to the outer query
* fix test
* reuse cloneWORecursiveSelects() in clone()
* fix subselect columns processing
This patch improves handling of NULLs in textual fields in ColumnStore.
Previously empty strings were considered NULLs and it could be a problem
if data scheme allows for empty strings. It was also one of major
reasons of behavior difference between ColumnStore and other engines in
MariaDB family.
Also, this patch fixes some other bugs and incorrect behavior, for
example, incorrect comparison for "column <= ''" which evaluates to
constant True for all purposes before this patch.
