scheduler: don't rely on boost interrupt on shutdown

Calling interrupt_all() will immediately stop the scheduler, so it's
safe to invoke stop() beforehand, and this removes the reliance on boost
to interrupt serviceQueue().

Cherry-picked from: 306f71b

src/init.cpp

@@ -216,6 +216,7 @@ void Shutdown()
     g_connman.reset();
 
     StopTorControl();
+
     UnregisterNodeSignals(GetNodeSignals());
     if (fDumpMempoolLater)
         DumpMempool();