Performance

- Corrected performance measurement.  The TDB2::gc requires data to
  be loaded first, but load time is measured separately.  This resulted
  in repeat counting.  Now the gc time subtracts any load time that is
  accumulated *during* the gc.  This is now a fair accounting of the time.

src/Timer.h

@@ -44,6 +44,7 @@ public:
   void start ();
   void stop ();
   unsigned long total () const;
+  void subtract (unsigned long);
 
 private:
   std::string    _description;