Remove duplicate ORDER BY/GROUP BY column references

Currently the optimizer doesn't cannot take into account duplicate column references in the GROUP/ORDER BY clauses. As a result, queries like:

select a col1, a col2 from t1 group by col1, col2;

will not use an index on t1(a), even though the duplicate columns
do not change the meaning of the query.

This is a simple task to remove duplicates in the order/group lists.
Care should be taken of queries with the ROLLUP modifier.

There is already an implementation attached to this task. The implementation has to be verified/changed so that:

• it takes care of queries with ROLLUP,
• it is verified that the place where duplicate removal is done covers all cases when it can and should be done,
• there is sufficient coverage with test cases.