Engine instructions (pause, fail, succeed, rollback)

We need to be able to specify special engine instructions under clauses 'on-error', 'on-success', 'on-complete' to perform a predefined set of commands:

* pause - To pause workflow execution
* fail - Immediately finish workflow execution with a failure
* succeed - Immediately complete workflow execution with success
* rollback - Rollback workflow

Example:

taskA:
  action: my.action
  on-success:
    - taskB: $.my_val = 10 # Here we start two other tasks as usually (fork)
    - taskC: $.my_val = 5
  on-error:
    - pause: $.my_val = 20 # Here we use a special engine instruction on-condition

For both tasks and instructions conditions are optional.

We also need to be able to define the same clauses (with a little bit different names for clarity such as 'on-task-error', 'on-task-success', 'on-task-complete') at workflow level to specify, for example, error handler for all task in one place. Corresponding clause specified for individual task takes precedence.