code: func (idx *CfIndex) entriesByBlockHashes(filterTypeKeys [][]byte,
  	filterType wire.FilterType, blockHashes []*chainhash.Hash) ([][]byte, error) {
  
  	if uint8(filterType) > maxFilterType {
  		return nil, errors.New("unsupported filter type")
  	}
  	key := filterTypeKeys[filterType]
  
  	entries := make([][]byte, 0, len(blockHashes))
  	err := idx.db.View(func(dbTx database.Tx) error {
  		for _, blockHash := range blockHashes {
  			entry, err := dbFetchFilterIdxEntry(dbTx, key, blockHash)
  			if err != nil {
  				return err
  			}
  			entries = append(entries, entry)
  		}
  		return nil
  	})
  	return entries, err
  }
docs: // entriesByBlockHashes batch fetches a filter index entry of a particular type
  // (eg. filter, filter header, etc) for a filter type and slice of block hashes.