so-5.5.0: Native support for std::chrono

Since v.5.5.0 it is possible to specify pause and period parameters for single_timer and schedule_timer methods as std::chrono-values:

class my_agent_t : public so_5::rt::agent_t
{
public :
  virtual void so_evt_start() override
  {
    m_hello_timer_id = so_environment()
       .schedule_timer(
          std::move( msg ),
          so_direct_mbox(),
          // Delay for a second.
          std::chrono::seconds(1),
          // Repeat every 1.25 of seconds.
          std::chrono::milliseconds(1250) );

    m_stop_timer_id = so_environment()
       .schedule_timer< msg_stop_signal >(
          m_shutdowner_mbox,
          // Delay for two seconds.
          std::chrono::seconds(2),
          // Not a periodic.
          std::chrono::seconds::zero() );
    ...
  }
  ...
private :
  so_5::timer_id_t m_hello_timer_id;
  so_5::timer_id_t m_stop_timer_id;
};

Old single_timer/schedule_timer are still here and can be used too. But they are marked as deprecated and can be removed in next major SObjectizer updates.