general: Use deducation guides for std::lock_guard and std::unique_lock

Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.

src/core/announce_multiplayer_session.cpp

@@ -65,14 +65,14 @@ void AnnounceMultiplayerSession::Stop() {
 
 AnnounceMultiplayerSession::CallbackHandle AnnounceMultiplayerSession::BindErrorCallback(
     std::function<void(const Common::WebResult&)> function) {
-    std::lock_guard<std::mutex> lock(callback_mutex);
+    std::lock_guard lock(callback_mutex);
     auto handle = std::make_shared<std::function<void(const Common::WebResult&)>>(function);
     error_callbacks.insert(handle);
     return handle;
 }
 
 void AnnounceMultiplayerSession::UnbindErrorCallback(CallbackHandle handle) {
-    std::lock_guard<std::mutex> lock(callback_mutex);
+    std::lock_guard lock(callback_mutex);
     error_callbacks.erase(handle);
 }
 
@@ -112,7 +112,7 @@ void AnnounceMultiplayerSession::AnnounceMultiplayerLoop() {
         UpdateBackendData(room);
         Common::WebResult result = backend->Update();
         if (result.result_code != Common::WebResult::Code::Success) {
-            std::lock_guard<std::mutex> lock(callback_mutex);
+            std::lock_guard lock(callback_mutex);
             for (auto callback : error_callbacks) {
                 (*callback)(result);
             }