box: make box.schema DDL functions transactional

Wraps multistatement DDL functions into begin/commit block if no
transaction is active.

The functions are:
- `box.schema.space.drop`
- `box.schema.index.create`
- `box.schema.index.drop`
- `box.schema.index.alter`
- `box.schema.sequence.drop`
- `box.schema.func.drop`
- `box.schema.user.create`
- `box.schema.user.drop`
- `box.schema.role.drop`

Added tests for atomicity of each transactioned function except
the `box.schema.role.drop`, which is implicitly tested with the
`box.schema.user.drop` test, and the `box.schema.index.drop`,
which is impossible to test in this flavor without using error
injection.

Updated the tests modified in tarantool/tarantool#8947, because
the space drop is atomic now.

Closes #4348

NO_DOC=bugfix