Make writing persistence.conf.bak more robust

… as suggested on #16461-note_13:

• I’ve checked and indeed, it would make sense to sync that file after copying.
• The chattr part is not needed as long as 1. t-p-s sync’s the file; 2. t-p-s is the only component that modifies that file (which is the case currently). But I guess it wouldn’t hurt and who knows, something else might start modifying that file at some point. Let’s do it.

Feature Branch: t-p-s:bugfix/16568-more-robust-backup, bugfix/16568-more-robust-backup

Parent Task: #10976

Related issues

• Related to #14544
• Related to #16461 (closed)
• Blocks #16209

Original created by @intrigeri on 16568 (Redmine)