DummyExpressionHolder

Update, delete and insert statement are not built with AST nodes, (for example Table[].update), although some portions of these statements (where clauses are sometimes built with it. The StatisticsListener needs to view every expression call as an AST, which is the reason for this class. AST are meant to be "non rendered", i.e. agnostic to specific DatabaseAdapter, this DummyExpressionHolder is an exception. TODO: unify expression building to be completely AST based.