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().

src/init.cpp

@@ -206,6 +206,7 @@ void Shutdown(NodeContext& node)
 
     // After everything has been shut down, but before things get flushed, stop the
     // CScheduler/checkqueue threadGroup
+    if (node.scheduler) node.scheduler->stop();
     threadGroup.interrupt_all();
     threadGroup.join_all();