Get the set of all preordered and revealed namespace hashes that have not expired.

def get_all_importing_namespace_hashes( self ):
        """
        Get the set of all preordered and revealed namespace hashes that have not expired.
        """
        cur = self.db.cursor()
        namespace_hashes = namedb_get_all_importing_namespace_hashes( cur, self.lastblock )
        return namespace_hashes