Stage 2 of MDEV-6152:

- Added mem_root to all calls to new Item
- Added private method operator new(size_t size) to Item to ensure that
  we always use a mem_root when creating an item.

This saves use once call to current_thd per Item creation

sql/opt_table_elimination.cc

@@ -1217,7 +1217,7 @@ void build_eq_mods_for_cond(THD *thd, Dep_analysis_context *ctx,
   }
   case Item_func::ISNULL_FUNC:
   {
-    Item *tmp=new Item_null(thd);
+    Item *tmp=new (thd->mem_root) Item_null(thd);
     if (tmp)
       check_equality(ctx, eq_mod, *and_level, cond_func, args[0], tmp);
     break;