Migrate from boost::date_time classes to the std::chrono classes

[fmauger]

From C++11 the standard chrono library provides a new set of classes to represent and manipulate time.
The preliminary version of Vire uses the Boost/date_time library with classes:

• boost::posix_time::ptime
• boost::posix_time::time_duration
• boost::posix_time::time_period
  As std now provide some standard tools, we should use them. This will make easier to use the std::thread library of which some features relies on chrono.