Move scheduler/threadGroup into common-init instead of per-app

This resolves #12229 which pointed out a shutdown deadlock due to
scheduler/checkqueue having been shut down while network message
processing is still running.

src/qt/bitcoin.cpp

@@ -28,7 +28,6 @@
 
 #include <init.h>
 #include <rpc/server.h>
-#include <scheduler.h>
 #include <ui_interface.h>
 #include <util.h>
 #include <warnings.h>
@@ -193,8 +192,6 @@ Q_SIGNALS:
     void runawayException(const QString &message);
 
 private:
-    boost::thread_group threadGroup;
-    CScheduler scheduler;
 
     /// Pass fatal exception message to UI thread
     void handleRunawayException(const std::exception *e);
@@ -300,7 +297,7 @@ void BitcoinCore::initialize()
     try
     {
         qDebug() << __func__ << ": Running initialization in thread";
-        bool rv = AppInitMain(threadGroup, scheduler);
+        bool rv = AppInitMain();
         Q_EMIT initializeResult(rv);
     } catch (const std::exception& e) {
         handleRunawayException(&e);
@@ -314,8 +311,7 @@ void BitcoinCore::shutdown()
     try
     {
         qDebug() << __func__ << ": Running Shutdown in thread";
-        Interrupt(threadGroup);
-        threadGroup.join_all();
+        Interrupt();
         Shutdown();
         qDebug() << __func__ << ": Shutdown finished";
         Q_EMIT shutdownResult();