Make namespace delete a more robust, two step process

1. On the filesystem, we should change the current delete to be a rename into a configurable trash directory, so it becomes an atomic operation.
2. In the MDS, we should add support for delete markers, as opposed to removing the entry altogether in from a blocking REST call.
3. We should also implement a background janitor service that cleans up items marked deleted in 1. and 2. above.
4. Once a namespace is marked deleted, it cannot be recreated until a janitor cleans it up (it is in a DELETING state up until that happens).

Eventually, we should do this for all entities in CDAP, but this ticket tracks it for namespaces only.