fixed UNION preparation


sql/item.cc:
  debug output added
sql/item.h:
  debug output added
sql/item_cmpfunc.cc:
  correct cleunup() for Item_in_optimizer
sql/item_cmpfunc.h:
  correct cleunup() for Item_in_optimizer
  debug output added
sql/item_func.h:
  debug output added
sql/item_subselect.cc:
  support of prepared statemnts added - mostly memorry allocation manegement, only one trabsformatio & correct cleupup()
sql/item_subselect.h:
  support of prepared statemnts added - mostly memorry allocation manegement, only one trabsformatio & correct cleupup()
sql/item_sum.cc:
  debug output added
sql/item_sum.h:
  debug output added
sql/sql_class.cc:
  function to switch allocation arena for Items
sql/sql_class.h:
  function to switch allocation arena for Items
  pointer on current prepared statement added
sql/sql_lex.cc:
  comment fixed
sql/sql_lex.h:
  item cleanup support
sql/sql_prepare.cc:
  - fixed preparation of PS to avoid storing junk in its memory + correct work with union
  - fixed tables cleanup for UNION & subqueries
sql/sql_select.cc:
  removed condition which is always true for now
  fixed layout
sql/sql_union.cc:
  support of UNION subquery cleanup
tests/client_test.c:
  test of repeatable subqueries
  test of correct UNION initialisation