- implement inheritance of sp_instr: public Query_arena.

We need every instruction to have its own arena, because we want to
  track instruction's state (INITIALIZED_FOR_SP -> EXECUTED). Because of 
  `if' statements and other conditional instructions used in stored 
  procedures, not every instruction of a stored procedure gets executed 
  during the first (or even subsequent) execution of the procedure. 
  So it's better if we track the execution state of every instruction 
  independently.
  All instructions of a given procedure now also share sp_head's 
  mem_root, but keep their own free_list.
  This simplifies juggling with free Item lists in sp_head::execute.
- free_items() moved to be a member of Query_arena. 
- logic of 'backup_arena' debug member of Query_arena has been
  changed to support
  multi-backups. Until now, TRUE 'backup_arena' meant that there is
  exactly one active backup of the THD arena. Now it means simply that
  the arena is used for backup, so that we can't accidentally overwrite an 
  existing backup. This allows doing multiple backups, e.g. in
  sp_head::execute and Cursor::fetch, when THD arena is already backed up
  but we want to set yet another arena (usually the 'permanent' arena,
  to save permanent transformations/optimizations of a parsed tree).

sql/sql_class.h

@@ -663,7 +663,10 @@ public:
   Item *free_list;
   MEM_ROOT *mem_root;                   // Pointer to current memroot
 #ifndef DBUG_OFF
-  bool backup_arena;
+  bool is_backup_arena; /* True if this arena is used for backup. */
+#define INIT_ARENA_DBUG_INFO is_backup_arena= 0
+#else 
+#define INIT_ARENA_DBUG_INFO  
 #endif
   enum enum_state
   {
@@ -681,12 +684,14 @@ public:
 
   Query_arena(MEM_ROOT *mem_root_arg, enum enum_state state_arg) :
     free_list(0), mem_root(mem_root_arg), state(state_arg)
-  {}
+  { INIT_ARENA_DBUG_INFO; }
   /*
     This constructor is used only when Query_arena is created as
     backup storage for another instance of Query_arena.
   */
-  Query_arena() {};
+  Query_arena() { INIT_ARENA_DBUG_INFO; }
+
+#undef INIT_ARENA_DBUG_INFO
   virtual Type type() const;
   virtual ~Query_arena() {};
 
@@ -726,6 +731,8 @@ public:
   void set_n_backup_item_arena(Query_arena *set, Query_arena *backup);
   void restore_backup_item_arena(Query_arena *set, Query_arena *backup);
   void set_item_arena(Query_arena *set);
+
+  void free_items();
 };