Suggestion for improving SelectedChunkMask (Columnar)

In columnar_reader.c in SelectedChunkMask() a call to predicate_refuted_by is made for each individual var. Because of this a where clause with an OR over multiple columns, like
```sql
WHERE a=1 OR b=1
```
will never filter out any chunks.
The solution is to put the min/max-predicates for all vars in a list and call predicate_refuted_by only once.
I implemented this for cstore_fdw (I have not been able to get the new citus extension running on my postgresql), see renevdzee/cstore_fdw@2c896d7 , and it seems to work and improves performance a lot.