Fixed possible deadlock in SolverMonitor

class SolverMonitor

{

public:

   SolverMonitor()

      : timeout_milliseconds( 500 ),

        stopped(true),

        started( false ),

        stopped( false ),

        timer( nullptr )

   {};

   ~SolverMonitor() {};

   {}

   virtual void refresh( bool force = false ) = 0;

   void runMainLoop()

   {

      stopped = false;

      // We need to use both 'started' and 'stopped' to avoid a deadlock

      // when the loop thread runs this method delayed after the

      // SolverMonitorThread's destructor has already called stopMainLoop()

      // from the main thread.

      started = true;

      const int timeout_base = 100;

      const std::chrono::milliseconds timeout( timeout_base );

            std::this_thread::sleep_for( timeout );

         }

      }

      // reset to initial state

      started = false;

      stopped = false;

   }

   void stopMainLoop()

      stopped = true;

   }

   protected:

   bool isStopped() const

   {

      return stopped;

   }

protected:

   double getElapsedTime()

   {

      if( ! timer )

   std::atomic_int timeout_milliseconds;

   std::atomic_bool started;

   std::atomic_bool stopped;

   Timer* timer;

} // namespace Solvers

} // namespace TNL