Make writing persistence.conf.bak more robust

Originally created by @intrigeri on #16568 (Redmine)

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

Edited May 15, 2020 by intrigeri

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