darksend: faster thread exit on shutdown

Signed-off-by: 216k155 <216k155@luxcore.io>

src/darksend.cpp

@@ -2098,7 +2098,7 @@ void ThreadCheckDarkSendPool() {
     unsigned int c = 0;
     std::string errorMessage;
 
-    while (true) {
+    while (!ShutdownRequested()) {
         c++;
 
         MilliSleep(2500);