Scale processing of "on-success", "on-error" and "on-complete"

When it comes to processing big "on-success", "on-error" and "on-complete" clauses of direct Mistral workflows, Mistral doesn't scale it which leads to having one engine busy for a long time (sometimes ~ maximum duration of a DB transaction) while others not doing anything.

In order to fix this problem, we may try to use the state WAITING not only for "join" tasks but also for tasks that have been created but not processed yet (not created all inputs, actions etc.)