Implement LogicalClock

class LogicalClock:

public:

      void setClusterTime(const LogicalTime& newTime);

      const LogicalTime& getClusterTime() const;

     // increments clusterTime and return the result. this method should sync with the wall clock 

     LogicalTime addTicks(unsigned ticks);

    // increments clusterTime but returns the time before the increment therefor "reserving" ticks.

    // Needed for the batch writes oplog functionality. this method should sync with the wall clock  

    LogicalTime reserveTicks(unsigned ticks);

  private:

      mutable stdx::mutex _mutex; // unless there is a measurable performance degradation 

                                                     // serialize on mutex

      LogicalTime _clusterTime;

 };

LogicalTime LogicalClock::reserveTicks(unsigned ticks) {

      const unsigned wallClockSecs = durationCount<Seconds>(

          getGlobalServiceContext()->getFastClockSource()->now().toDurationSinceEpoch());

      stdx::lock_guard<stdx::mutex> lock(_mutex);

      // Optimistic assumption that seconds did not change

      LogicalTime currentTime = _clusterTime;

     _clusterTime.addTicks(ticks);

      unsigned currentSecs = _clusterTime.asTimestamp().getSecs();

      // If the seconds need to be updated, try to do it. This can happen at most once per second.

      if (MONGO_unlikely(currentSecs < wallClockSecs)) {

          // First fix the seconds portion. 

         _clusterTime = LogicalTime(Timestamp(wallClockSecs, 1));

          currentTime = _clusterTime;

          _clusterTime.addTicks(ticks);

      }

      return currentTime;

  }