[sql generation] fix missing join with outer joins w/ rhs invariant variable... ... where the principal where not the processed relation. This led to major refactoring in outer join handling, which is now much clearer and powerful. Closes #1579228

[test, cw props] ensure we can't add user specific / system-wide properties we had this pb on an historical instance in our intranet (eg. running for about 8 years).

[ms planner] fix buggy temp table name generation, may generate duplicated unique id with subqueries/union