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