Support more than one CScheduler thread for serial clients

This will be used by CValidationInterface soon.

This requires a bit of work as we need to ensure that most of our
callbacks happen in-order (to avoid synchronization issues in
wallet) - we keep our own internal queue and push things onto it,
scheduling a queue-draining function immediately upon new
callbacks.

src/validationinterface.h

@@ -75,8 +75,6 @@ private:
     friend void ::UnregisterAllValidationInterfaces();
 
 public:
-    CMainSignals();
-
     /** Register a CScheduler to give callbacks which should run in the background (may only be called once) */
     void RegisterBackgroundSignalScheduler(CScheduler& scheduler);
     /** Unregister a CScheduler to give callbacks which should run in the background - these callbacks will now be dropped! */