txn: fix execution order of commit triggers

Both commit and rollback triggers are currently added to the list head.
As a result, they are both run in the reverse order. This is correct for
rollback triggers, because this matches the order in which statements
that added the triggers are rolled back, but this is wrong for commit
triggers. For example, suppose we create a space and then create an
index for it in the same transaction. We expect that on success we first
run the trigger that commits the space and only then the trigger that
commits the index, not vice versa. That said, reverse the order of
commit triggers in the scope of preparations for transactional DDL.