MDEV-7951 - sql_alloc() takes 0.25% in OLTP RO

sql_alloc() has additional costs compared to direct mem_root allocation:
- function call: it is defined in a separate translation unit and can't be
  inlined
- it needs to call pthread_getspecific() to get THD::mem_root

It is called dozens of times implicitely at least by:
- List<>::push_back()
- List<>::push_front()
- new (for Sql_alloc derived classes)
- sql_memdup()

Replaced lots of implicit sql_alloc() calls with direct mem_root allocation,
passing through THD pointer whenever it is needed.

Number of sql_alloc() calls reduced 345 -> 41 per OLTP RO transaction.
pthread_getspecific() overhead dropped 0.76 -> 0.59
sql_alloc() overhed dropped 0.25 -> 0.06

sql/sql_delete.cc

@@ -496,7 +496,7 @@ bool mysql_delete(THD *thd, TABLE_LIST *table_list, COND *conds,
       Filesort_tracker *fs_tracker= 
         thd->lex->explain->get_upd_del_plan()->filesort_tracker;
 
-      if (!(sortorder= make_unireg_sortorder(order, &length, NULL)) ||
+      if (!(sortorder= make_unireg_sortorder(thd, order, &length, NULL)) ||
 	  (table->sort.found_records= filesort(thd, table, sortorder, length,
                                                select, HA_POS_ERROR,
                                                true,