ddl: transactional DDL does not care about savepoints

[Gerold103]

box.cfg{}
function ddl()
	local check1, check2, check3, check4
	local s1 = box.schema.create_space('test1')
	local save = box.savepoint()
	local s2 = box.schema.create_space('test2')
	check1 = box.space.test1 ~= nil
	check2 = box.space.test2 ~= nil
	box.rollback_to_savepoint(save)
	check3 = box.space.test1 ~= nil
	check4 = box.space.test2 ~= nil
	return check1, check2, check3, check4
end

res = nil
box.begin() res = {ddl()} box.commit()
res

Expected output, is that check4 is false, other checks are true. But reality is disappointing:

tarantool> res
---
- - true
  - true
  - true
  - true
...

[Gerold103]

I thought, that I will fix that, but looks like Vova now is assigned. Nonetheless here are my thoughts on what I had planned to do. Perhaps, it will be useful.

The problem is that we can't set on_rollback triggers on individual statements. But we don't need to, strictly speaking. There are only 2 ways, as I know, how a statement can be rolled back:

1. rollback the whole transaction;
2. rollback to a savepoint.

All cases like rollback on yield, rollback on conflict, voluntary rollback, rollback on an error from WAL, are covered by the case one.
It means, that we don't need to have on_rollback triggers per statement - it would be too expensive. It is enough to have them for txn and for savepoint. Alter should set triggers on rollback to the most recent savepoint.