ModeShape should support the '<>' comparison operator in JCR-SQL2

It was an oversight that support for this "not-equal-to" operator was not included with the rest of JCR-SQL2.

(Note that we already support the non-standard "!=" operator, so we should be able to pretty easily add support for another operator with equivalent semantics. However, we should convert the non-standard "!=" to the standard "<>" in all queries.)