Class AbstractTransactionalQuery should be public (#12285)

AbstractTransactionalQuery was not visible outside its package
(default visibility). Added `public`.

Making this class public avoids lots of boilerplate code when making
alternate implementations of a QueryDelegate.

A standard TableQuery does a lot of introspection on the DB schema. 
If the schema is known beforehand, alternate implementations may specify
this information explicitly, and inherit from AbstractTransactionalQuery
to reuse part of the implementation.

Change-Id: I7d6b399ed7c755941ee9be7931a2ba80e812b5ab

server/src/com/vaadin/data/util/sqlcontainer/query/AbstractTransactionalQuery.java

@@ -31,7 +31,7 @@ import com.vaadin.data.util.sqlcontainer.connection.JDBCConnectionPool;
  * @author Vaadin Ltd
  * @since 6.8.9
  */
-abstract class AbstractTransactionalQuery implements Serializable {
+public abstract class AbstractTransactionalQuery implements Serializable {
 
     private JDBCConnectionPool connectionPool;
     private transient Connection activeConnection;