Fix gravity swapping

[DL6ER]

What does this implement/fix?

Swapping the databases must be the last step before the cleanup. Otherwise, FTL may be reloading from an only partially completed database causing spurious errors and/or warnings.

This fixes occasionally seen messages when pihole -g is run (either manually or automatically) such as

2023-10-22 03:12:08.093 [80311/T80329] INFO: Gravity database has been updated, reloading now
2023-10-22 03:12:08.096 [80311/T80329] ERR: gravityDB_count(SELECT value FROM info WHERE property = 'gravity_count';) - SQL error step no more rows available
2023-10-22 03:12:08.096 [80311/T80329] WARNING: Count of gravity domains not available. Please run pihole -g
2023-10-22 03:12:08.121 [80311/T80329] INFO: Compiled 0 allow and 4 deny regex for 20 clients in 24.0 msec

happening because the new database was already swapped into place before the corresponding numbers have been counted and written to the database.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pihole-up-destroy-correct-number-of-domains-on-adlist/65665/8