Support for vtgate -enable_direct_ddl flag

[shlomi-noach]

Description

We add a -enable_direct_ddl (bool) flag to vtgate. The default is true.
It is now possible to disable direct DDL via vtgate. If you run vtgate -enable_direct_ddl=false, then, for example:

$ mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1

mysql> select @@ddl_strategy;
+----------------+
| @@ddl_strategy |
+----------------+
| direct         |
+----------------+
1 row in set (0.00 sec)

mysql> create table my_test(id int primary key);
ERROR 1105 (HY000): direct DDL is disabled

This variable only controls direct (non-online) DDLs. As reminder, online DDL can be controlled via -enable_online_ddl. It is possible to enable one and disable another, enable both, or disable both.

Checklist

• Tests were added or are not required
• Documentation was added or is not required

cc @piki @deepthi @mavenraven

[shlomi-noach]

This change only affects vtgate. Schema changes via vtctl ApplySchema are unaffected by this flag.

[systay]

Would be nice if you could add a test

[shlomi-noach]

re: test, I agree; but the only way I can see it is if I spawn a dedicated new endtoend test just for this, becaus eit entails starting vtgate with the new flag. Any thoughts?

[systay]

well, how about the planner not directly depending on the flag, and have this value be given to the planner from the outside.
this makes the dependency explicit and makes it easier to test. WDYT?