Rename:

- current_arena to stmt_arena: the thread may have more than one
'current' arenas: one for runtime data, and one for the parsed 
tree of a statement. Only one of them is active at any moment.
- set_item_arena -> set_query_arena, because Item_arena was renamed to 
Query_arena a while ago
- set_n_backup_item_arena -> set_n_backup_active_arena;
the active arena is the arena thd->mem_root and thd->free_list
are currently pointing at.
- restore_backup_item_arena -> restore_active_arena (with the same
rationale)
- change_arena_if_needed -> activate_stmt_arena_if_needed; this
method sets thd->stmt_arena active if it's not done yet.

sql/sql_parse.cc

@@ -5275,7 +5275,7 @@ mysql_new_select(LEX *lex, bool move_down)
     it's a constant one. The flag is switched off in the end of
     mysql_stmt_prepare.
   */
-  if (thd->current_arena->is_stmt_prepare())
+  if (thd->stmt_arena->is_stmt_prepare())
     select_lex->uncacheable|= UNCACHEABLE_PREPARE;
   if (move_down)
   {
@@ -6081,7 +6081,7 @@ TABLE_LIST *st_select_lex::add_table_to_list(THD *thd,
     ptr->db= empty_c_string;
     ptr->db_length= 0;
   }
-  if (thd->current_arena->is_stmt_prepare_or_first_sp_execute())
+  if (thd->stmt_arena->is_stmt_prepare_or_first_sp_execute())
     ptr->db= thd->strdup(ptr->db);
 
   ptr->alias= alias_str;
@@ -7301,7 +7301,7 @@ bool create_table_precheck(THD *thd, TABLE_LIST *tables,
       against the opened tables to ensure we don't use a table that is part
       of the view (which can only be done after the table has been opened).
     */
-    if (thd->current_arena->is_stmt_prepare_or_first_sp_execute())
+    if (thd->stmt_arena->is_stmt_prepare_or_first_sp_execute())
     {
       /*
         For temporary tables we don't have to check if the created table exists