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.cpp

@@ -105,4 +105,12 @@ unsigned long Timer::total () const
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+void Timer::subtract (unsigned long value)
+{
+  if (value > _total)
+    _total = 0;
+  else
+    _total -= value;
+}
 
+////////////////////////////////////////////////////////////////////////////////