Performance Timing

- Change the was performance measurements are taken.  This in preparation
  to automated daily perf charts, leading to efforts to speed up certain
  operations.

src/TDB2.h

@@ -92,6 +92,7 @@ public:
   void add (const Task&);
   void modify (const Task&);
   void commit ();
+  void synch ();
   int  gc ();
   int  next_id ();