keepResults option in NNThroughputBenchmark should call saveNamespace()

-keepResults option is usually used to make it possible to rerun NNThroughputBenchmark with existing namespace state -useExisting. E.g. first generate files with create command and then run fileStatus command on generated files.
NNThroughputBenchmark should call saveNamespace() when -keepResults option is specified. Otherwise NN startup takes a while since it needs to digest large edits file starting from the empty image.