Make TDB2.backlog non-public

The stats command gets this information from an API that will also work
for TaskChampion.  The sync command still accesses the field directly,
as the command must be completely rewritten for TaskChampion.

src/TDB2.h

@@ -135,6 +135,9 @@ public:
   // Read-only mode.
   bool read_only ();
 
+  int num_local_changes ();
+  size_t data_size ();
+
   void clear ();
   void dump ();
 
@@ -152,6 +155,9 @@ public:
   TF2 pending;
   TF2 completed;
   TF2 undo;
+
+protected:
+  friend class CmdSync; // CmdSync accesses the backlog directly
   TF2 backlog;
 
 private: