[REVIEW REQUEST] CALCITE-5253 NATURAL JOIN erroneous validation.

[stanilovsky evgeny]

Hi,

I would like to request a review for CALCITE-5253, pr [1], it fixes  
partially erroneous natural join validaton logic.
Thanks !
[1] https://github.com/apache/calcite/pull/2889
[2] https://issues.apache.org/jira/browse/CALCITE-5253