Add Query bean filterMany() taking a closure

Query beans also generate a "Assoc" query bean that is used
for the relationships. Previously these were generated into a
.query.assoc package and with this they are generated as an
inner class of the query bean.

This then allows the change where an additional filterMany()
is generated on the "Assoc" bean that takes a closure. This
simplifies the use of query beans for filterMany.

This change to the generated query beans needs a change to the
ebean-agent enhancement.

ebean-api/src/main/java/io/ebean/ExpressionFactory.java

@@ -36,6 +36,11 @@ import java.util.Map;
  */
 public interface ExpressionFactory {
 
+  /**
+   * Return a new ExpressionList.
+   */
+  <T> ExpressionList<T> expressionList();
+
   /**
    * Path exists - for the given path in a JSON document.
    */