txn: undo commit/rollback triggers when reverting to savepoint

When reverting to a savepoint inside a DDL transaction, apart from
undoing changes done by the DDL statements to the system spaces, we also
have to

 - Run rollback triggers installed after the savepoint was set, because
   otherwise changes done to the schema by DDL won't be undone.
 - Remove commit triggers installed after the savepoint, because they
   are not relevant anymore, apparently.

To achieve that let's append DDL triggers right to txn statements.
This allows us to easily discard commit triggers and run rollback
triggers when a statement is rolled back.

Note, txn commit/rollback triggers are not removed, because they are
still used by applier and Lua box.on_commit/on_rollback functions.

Closes #4364
Closes #4365