ExecutionPlan#progress

Ability to calculate execution progress for the plan. Every action can
specify:

* +run_progress_done+/+finalize_progress_done+ - should return 0..100
  it's called only when the action is in running/suspended state.

* +run_progress_weight+/+finalize_progress_weight+ - to make the
  action more significant in the progress of the plan (1 by default)