check_serialization_api_v_1

check_serialization_api_v_2

The difference from version 1:

* when the process is finished the field "_end" appear
* after every pass_milestone() in the object _miliestone_pass gets the
  current time (and _miliestone_pass is stored in serialized object)

check_serialization_api_v_3

The difference from version 2:

* Added _is_paused and _elapsed.