[schema] improve normalization of RQLExpressions Parse and print back the expression instead of manipulating the string. Among other benefits, it means we don't mangle embedded string constants that contain commas or multiple spaces. Closes #6694426

[server/rql2sql] use VariableRef.is_equivalent explicitly instead of relying on __eq__ What we want to compare is the variable referenced by the VariableRef, we should make that explicit. Closes #6694406