MODE-1671 Added 'mode:id' pseudocolumn for JCR-SQL2 queries

It is now possible to use the 'mode:id' pseudocolumn that exists on all selectors
to obtain the javax.jcr.Node.getIdentifier() value. It can be used in WHERE constraints
and JOIN criteria.
Merge pull request #1184 from rhauch/mode-1671

MODE-1671 Added 'mode:id' pseudocolumn for JCR-SQL2 queries