Tweak DB counter update

[DL6ER]

What does this implement/fix?

Improve database updating procedure by avoiding opening a dedicated / separate database connection for updating the query counters when we already have the disk database attached to our in-memory database under the disk.* namespace. This also brings a bit of code duplication and removal of functions called only once.

I noticed this in it in this - otherwise unrelated - Discourse topic:
https://discourse.pi-hole.net/t/pihole-v6-on-low-spec-hardware/72442/5?u=dl6er
where we're seeing

2024-09-17 20:17:11.635 CEST [1112/T1128] DEBUG_DATABASE: Opening FTL database in export_queries_to_disk() (/root/FTL/src/database/query-table.c:696)

which is unnecessary here. This PR is also a performance improvement, even when this is not immediately relevant for DNS operation, it is still meaningful as the on-disk database will be blocked for a bit less of time.

This PR also fixes a small bug which adds too many queries to the database counters.

---

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

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

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.