UNION operator should produce a key-free schema

[KinanBab]

We cannot guarantee that a UNION operator will respect the key of either input, e.g. a diamond flow may produce the same record with the same key twice, once from every arm of the diamond.

Implement this in UNION operator and test it with diamond with different keys, e.g. secondary index in hotcrp schema for DocumentLink with Paper.leadContantId set to an OWNER.