code: @Nonnull
      public String createLockName( @Nonnull final String sessionId ) {
          if ( sessionId == null ) {
              throw new IllegalArgumentException( "The sessionId must not be null." );
          }
          return "lock:" + _storageKeyFormat.format(sessionId);
      }
docs: Creates the name/key that can be used for the lock stored in memcached.
  @param sessionId the session id for that a lock key shall be created.
  @return a String.