code: File findFinalizedEditsFile(long startTxId, long endTxId)
    throws IOException {
      File ret = findFile(NameNodeDirType.EDITS,
          getFinalizedEditsFileName(startTxId, endTxId));
      if (ret == null) {
        throw new IOException(
            "No edits file for txid " + startTxId + "-" + endTxId + " exists!");
      }
      return ret;
    }
docs: Return the first readable finalized edits file for the given txid.