Tasks should have an "canceled" state in their lifecycle

It is useful in multiple cases to have such a state. In the scope of this jira ticket, there is also the fact of using it everywhere it should:

• when someone cancels a workflow, all open tasks should go to "canceled"
• when a workflow goes to a "stop" node while there are some open tasks in some places, tasks should go to "open tasks"
• when a workflow goes to a merge node and the merge expects one branch to arrive for going further , all open tasks of the nodes in the branch that was pending should go to canceled tasks
• in the future, when in the n-task node, we only wait for x% to be processed, not processed tasks should to to canceled state