@Override
  public String getBackupFileName(String fileName, int backupIndex) {
    return fileName + ".bak." + backupIndex;
  }
An abstract {@link BackupStrategy2}, simply append '.bak.n' to the end of normal file name when
naming a backup with index n.
<p>
Developers can simply extend this class when defining their own {@link BackupStrategy2}.

@since 1.9.0
xlog/src/main/java/com/elvishew/xlog/printer/file/backup/AbstractBackupStrategy.java