Using a constrained Name for namespaces names

This is in preparation of the `DirPath/FilePath` update.
Yeh, I agree with this. And further, I think we are using unsafe way too much. I don't think we should be assuming so much, even if it is coming from thrift etc... we should handle the failure case every time. The original premise for the unsafe methods was to be used on the MR jobs for perf reasons - not to make some claim that it is really right. And I think we should do a review and get back to that.