Backup persistence.conf before modifying it in t-p-s

Originally created by @intrigeri on #16461 (Redmine)

Even without the restore code suggested there, saving a backup should already be a net win:

for users who might notice the “.bak” file and recover themselves
for help desk when they help users recover from #10976 (closed)
for me when I get debugging data about #10976 (closed) (depending on the mtime of the backup file, I’ll be able to tell for sure if t-p-s was used at all recently, regardless of what users remember: nothing else modifies this backup file).

Feature Branch: bugfix/16461-lost-persistence-config, persistence-setup:bugfix/16461-lost-persistence-config

Edited Jan 21, 2021 by intrigeri

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