SystematicsHandles: Add SysWriteDecorHandle::lock().

We should be locking decorations after we make them. SysWriteDecorHandle wasn't doing that. The design, however, isn't compatible with the usual way of doing this automatically via RAII. The best we can do without major changes seems to be to add a lock() method that then needs to be called explicitly.

See ATLASRECTS-8008.