code: @Nonnull
      public String createBackupKey( @Nonnull final String origKey ) {
          if ( origKey == null ) {
              throw new IllegalArgumentException( "The origKey must not be null." );
          }
          return BACKUP_PREFIX + _storageKeyFormat.format(origKey);
      }
docs: Creates the name/key that is used for the data (session or validity info)
  that is additionally stored in a secondary memcached node for non-sticky sessions.
  @param origKey the session id (or validity info key) for that a key shall be created.
  @return a String.