Naive DB state handling and no way to reload the DB state in transaction

[a3kov]

The current implementation is naive - its assuming that DB state is equal to loaded (runtime) state. A correct implementation would need to assure that with some form of locking. For example, before transitioning, we could lock the row (via a write lock or a simple advisory lock) and re-read the state column, making sure our copy of it is up to date.

With non-transactional transition its ignored completely, but maybe that is fine for people who use it (????).
Now, a user of the library might think it's possible to achieve correctness with transition_multi. However, it has no way to accept the loaded schema inside transaction, and doesn't fetch the record itself. It accepts the schema argument which must be known (loaded) outside of transaction context.

An alternative approach would be to use a transition query with WHERE state=^previous_state, but then it wouldn't work with a changeset.