Review request for CALCITE-1912: Support "FOR SYSTEM_TIME AS OF" in regular queries

[Francis Chuang]

Hey all,

There is a PR that has sort of been forgotten and the contributor would 
like to have it reviewed.

Can someone please review it?

Link to PR: https://github.com/apache/calcite/pull/759
JIRA issue: https://issues.apache.org/jira/browse/CALCITE-1912

Francis