Refactor display node logic

DisplayNode logic duplicates Teiid SQLStringVisitor logic and quickly gets out of synch. There are also several pending changes (nested table syntax, named parameter syntax) that could have their fixes obviated by just changing the display node logic to use the SQLStringVisitor to construct the display node tree.

The bulk of the display node classes can be removed since they contain no logic and could be replaced with instanceof checks on the underlying language object instead.

Teiid's SQLStringVisitor will need some modifications to accommodate this change, which will get rolled into 7.1.1