TDB2 & CmdCustom

- Converted CmdCustom to use TDB2, which means that for the first time,
  2.0 is now optimizing data load based on filter characteristics.  This
  means that "task list" and "task next" etc are now *fast*.
- Improved diagnostics for TDB2, for debugging.

src/commands/CmdCustom.cpp

@@ -92,17 +92,10 @@ int CmdCustom::execute (std::string& output)
   context.a3.dump ("CmdCustom::execute");
 
   // Load the data.
-  // TODO Replace with TDB2.
-  std::vector <Task> tasks;
-  context.tdb.lock (context.config.getBoolean ("locking"));
   handleRecurrence ();
-  context.tdb.load (tasks);
-  context.tdb.commit ();
-  context.tdb.unlock ();
-
-  // Apply filter.
+  context.tdb2.commit ();
   std::vector <Task> filtered;
-  filter (tasks, filtered);
+  filter (filtered);
 
   // Sort the tasks.
   std::vector <int> sequence;