MDEV-7004 - Merge scalability fixes from 10.0-power

Preallocate dynamic array on THD mem_root to avoid expensive malloc.

sql/sql_select.cc

@@ -5247,7 +5247,8 @@ update_ref_and_keys(THD *thd, DYNAMIC_ARRAY *keyuse,JOIN_TAB *join_tab,
   /* set a barrier for the array of SARGABLE_PARAM */
   (*sargables)[0].field= 0; 
 
-  if (my_init_dynamic_array(keyuse,sizeof(KEYUSE),20,64,
+  if (my_init_dynamic_array2(keyuse, sizeof(KEYUSE),
+                             thd->alloc(sizeof(KEYUSE) * 20), 20, 64,
                              MYF(MY_THREAD_SPECIFIC)))
     return TRUE;