Adds a Batch id to the pending cache, with its transaction ids.

        Args:
            batch (str): The id of the pending batch

def notify_batch_pending(self, batch):
        """Adds a Batch id to the pending cache, with its transaction ids.

        Args:
            batch (str): The id of the pending batch
        """
        txn_ids = {t.header_signature for t in batch.transactions}
        with self._lock:
            self._pending.add(batch.header_signature)
            self._batch_info[batch.header_signature] = txn_ids
            self._update_observers(batch.header_signature,
                                   ClientBatchStatus.PENDING)