Register the subscriber for the given event subscriptions.

        Raises:
            InvalidFilterError
                One of the filters in the subscriptions is invalid.

def add_subscriber(self, connection_id, subscriptions,
                       last_known_block_id):
        """Register the subscriber for the given event subscriptions.

        Raises:
            InvalidFilterError
                One of the filters in the subscriptions is invalid.
        """
        with self._subscribers_cv:
            self._subscribers[connection_id] = \
                EventSubscriber(
                    connection_id, subscriptions, last_known_block_id)

        LOGGER.debug(
            'Added Subscriber %s for %s', connection_id, subscriptions)