MDEV-20494 ER_NOT_FORM_FILE or assertion upon adding partition to period table

- Fixed mysql_prepare_create_table() constraint duplicate checking;
- Refactored period constraint handling in mysql_prepare_alter_table():
  * No need to allocate new objects;
  * Keep old constraint name but exclude it from dup checking by automatic_name;
- Some minor memory leaks fixed;
- Some conceptual TODOs.

sql/sql_partition.cc

@@ -6875,6 +6875,7 @@ static void alter_partition_lock_handling(ALTER_PARTITION_PARAM_TYPE *lpt)
       thd->set_stmt_da(&tmp_stmt_da);
     }
 
+    // TODO: why error status of reopen_tables() is ignored?
     if (unlikely(thd->locked_tables_list.reopen_tables(thd, false)))
       sql_print_warning("We failed to reacquire LOCKs in ALTER TABLE");