dnfdaemon: resolve and do_transaction cannot run simultaneously

Since resolve() method re-sets transaction, use transaction mutex to
prevent running Goal.resolve() if another transaction is running.

dnf5daemon-server/services/goal/goal.cpp

@@ -121,6 +121,13 @@ void Goal::dbus_register() {
 }
 
 sdbus::MethodReply Goal::resolve(sdbus::MethodCall & call) {
+    auto & transaction_mutex = session.get_transaction_mutex();
+    if (!transaction_mutex.try_lock()) {
+        //TODO(mblaha): use specialized exception class
+        throw std::runtime_error("Cannot acquire transaction lock (another transaction is running).");
+    }
+    std::lock_guard<std::mutex> transaction_lock(transaction_mutex, std::adopt_lock);
+
     // read options from dbus call
     dnfdaemon::KeyValueMap options;
     call >> options;