code: func (d *DeliverClient) GetOldestBlock() (*cb.Block, error) {
  	err := d.seekOldest()
  	if err != nil {
  		return nil, errors.WithMessage(err, "error getting oldest block")
  	}
  
  	return d.readBlock()
  }
docs: // GetOldestBlock gets the oldest block from a peer/orderer's deliver service