Added a class to measure the duration of atomic operations

This would simplify the way measure atomic opeation duration
in certain cases. For example if we want to get the duration
for each operation which runs in an iteration

    for i in range(repetitions):
       with scenario_utils.AtomicAction(instance_of_base_scenario_subclass,
                                        "name_of_action"):
           self.clients(<client>).<operation>

Change-Id: I48793fc0af160656f37964ca5d4e9b59b62de685
Merge "Added a class to measure the duration of atomic operations"