Added critical section to FMultiRegistry to make it thread-safe

[tinyogre]

Various login tasks access FMultiRegistry, which contains a static TMap<>. Those tasks run on worker threads, meaning that TMap<> was being both read from and modified by multiple threads at once, especially when logging in two PIE clients at the same time. This adds a critical section around all accesses to the map to make FMultiRegistry thread-safe.