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_derived.cc

@@ -407,7 +407,7 @@ bool mysql_derived_merge(THD *thd, LEX *lex, TABLE_LIST *derived)
     if (!derived->get_unit()->prepared)
     {
       dt_select->leaf_tables.empty();
-      make_leaves_list(dt_select->leaf_tables, derived, TRUE, 0);
+      make_leaves_list(thd, dt_select->leaf_tables, derived, TRUE, 0);
     } 
 
     derived->nested_join= (NESTED_JOIN*) thd->calloc(sizeof(NESTED_JOIN));