Logging: Don't lock the queue for the duration of the write

src/common/logging/backend.cpp

@@ -83,8 +83,10 @@ private:
                 }
             };
             while (true) {
+                {
                     std::unique_lock<std::mutex> lock(message_mutex);
                     message_cv.wait(lock, [&] { return !running || message_queue.Pop(entry); });
+                }
                 if (!running) {
                     break;
                 }