Make engine transactions shorter

Make engine transactions shorter. Now direct workflow handler has a pretty heavy DB query to find all completed tasks we haven’t scheduled next tasks for. Theoretically we can optimize it by passing “cause task” to corresponding workflow controller methods and do search only for this task. There may be potential problems with “join” though, need to think how to handler it properly in this case.

In general, we need to look at transactions and try to move out of their scope as much logic as possible, do all possible preprocessing before transactions.