Werk #11247: Preserve piggyback data during restart and updates

For performance reasons, the Piggyback data is stored in the tmpfs of the site (in tmp/check_mk/piggyback). This has the disadvantage that the files disappear as soon as the checkmk server is restarted or the tmpfs is unmounted (e.g. during an update).

The lack of piggyback data can temporarily lead to incorrect check results and, if you use the Dynamic Configuration, even cause certain hosts/services to disappear from the configuration for a short time.

With this change the content of tmp/check_mk/piggyback and tmp/check_mk/piggyback_sources is saved to var/omd/tmpfs-dump.tar during certain actions (currently: omd stop, omd umount).

If the "omd start" detects that tmpfs needs to be reinitialized, the backup from var/omd/tmpfs-dump.tar is restored to the fresh tmpfs.

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