Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/scheduler.cpp

@@ -174,7 +174,7 @@ void SingleThreadedSchedulerClient::ProcessQueue() {
     // to ensure both happen safely even if callback() throws.
     struct RAIICallbacksRunning {
         SingleThreadedSchedulerClient* instance;
-        RAIICallbacksRunning(SingleThreadedSchedulerClient* _instance) : instance(_instance) {}
+        explicit RAIICallbacksRunning(SingleThreadedSchedulerClient* _instance) : instance(_instance) {}
         ~RAIICallbacksRunning() {
             {
                 LOCK(instance->m_cs_callbacks_pending);