scripted-diff: Rename MainSignalsInstance() class to MainSignalsImpl()

```
-BEGIN VERIFY SCRIPT-
s() { git grep -l "$1" src test doc | xargs sed -i "s/$1/$2/g"; }
s 'MainSignalsInstance' 'MainSignalsImpl'
-END VERIFY SCRIPT-

src/validationinterface.cpp

@@ -17,14 +17,14 @@
 #include <utility>
 
 /**
- * MainSignalsInstance manages a list of shared_ptr<CValidationInterface> callbacks.
+ * MainSignalsImpl manages a list of shared_ptr<CValidationInterface> callbacks.
  *
  * A std::unordered_map is used to track what callbacks are currently
  * registered, and a std::list is used to store the callbacks that are
  * currently registered as well as any callbacks that are just unregistered
  * and about to be deleted when they are done executing.
  */
-class MainSignalsInstance
+class MainSignalsImpl
 {
 private:
     Mutex m_mutex;
@@ -42,7 +42,7 @@ public:
     // our own queue here :(
     SingleThreadedSchedulerClient m_schedulerClient;
 
-    explicit MainSignalsInstance(CScheduler& scheduler LIFETIMEBOUND) : m_schedulerClient(scheduler) {}
+    explicit MainSignalsImpl(CScheduler& scheduler LIFETIMEBOUND) : m_schedulerClient(scheduler) {}
 
     void Register(std::shared_ptr<CValidationInterface> callbacks)
     {
@@ -94,7 +94,7 @@ static CMainSignals g_signals;
 void CMainSignals::RegisterBackgroundSignalScheduler(CScheduler& scheduler)
 {
     assert(!m_internals);
-    m_internals = std::make_unique<MainSignalsInstance>(scheduler);
+    m_internals = std::make_unique<MainSignalsImpl>(scheduler);
 }
 
 void CMainSignals::UnregisterBackgroundSignalScheduler()

src/validationinterface.h

@@ -175,10 +175,10 @@ protected:
     friend class ValidationInterfaceTest;
 };
 
-class MainSignalsInstance;
+class MainSignalsImpl;
 class CMainSignals {
 private:
-    std::unique_ptr<MainSignalsInstance> m_internals;
+    std::unique_ptr<MainSignalsImpl> m_internals;
 
     friend void ::RegisterSharedValidationInterface(std::shared_ptr<CValidationInterface>);
     friend void ::UnregisterValidationInterface(CValidationInterface*);