code: func createPurgeIndexByHeightRangeEndKey(blockHeight uint64) []byte {
  	var endKey []byte
  	endKey = append(endKey, purgeIndexByHeightPrefix)
  	endKey = append(endKey, compositeKeySep)
  	endKey = append(endKey, util.EncodeOrderPreservingVarUint64(blockHeight)...)
  	endKey = append(endKey, byte(0xff))
  	return endKey
  }
docs: // createPurgeIndexByHeightRangeEndKey returns a endKey to do a range query on index stored in transient store
  // using blockHeight