- The QEP was not properly modified to use the special
   'linked'(aka. pushed) read functions since the order of
   ha_make_pushed_join() and pick_table_access_method() had
   been changed.
 - Create a new Abstract_query_plan function which the handler
   implementation can call to modify the table access method
   used by QEP_TAB.
 - Modify ndbcluster_make_pushed_join() to call the new function
   after the pushed joins has been defined. Thus the
   'linked' read functions will be called when the QEP is executed.
 - Improve the comments describing ha_make_pushed_join() and the
   responsibility of the handler.
 - Remove double call to plan_is_const()
 - Add new ndbcluster.pushed_join test which checks that the
   integration of pushed join functionality works as expected
   when MySQL Server is compiled with the NDB storage engine.