There should be a way to limit workflow executions

[Xavier Hardy]

It is possible to overload the messaging queues, engines and executors, using the following techniques:

- workflow task loops

tasks:
  looped_task:
    on-success:
      - looped_tasks

- with-items with an unexpected list length

tasks:
  looped_task:
    with-items: some_item in <% $.some_very_long_list %>
    action: std.echo
    # the faster the action to execute, the higher the chances to block the system

- high number of concurrent executions

tasks:
  looped_task:
    concurrency: 10000
    with-items: some_item in <% $.some_very_long_list %>
    action: std.echo
    # the faster the action to execute, the higher the chances to block the system

To prevent this, we must be able to set limits (or to change them if they exists but are hard-coded) to the following items for Mistral globally (not only for a particular workflow):
- task recursion limit (maximum action/workflow executions per task maybe?)
- maximum item list size
- maximum concurrency