What does this implement/fix?

Add new config option database.useWAL defaulting to true making the use of the WAL journal optional for the on-disk query database. Reasons why this might be useful are given in the description of the new config option:

Should FTL enable Write-Ahead Log (WAL) mode for the on-disk query database (configured via files.database)? It is recommended to leave this setting enabled for performance reasons. About the only reason to disable WAL mode is if you are experiencing specific issues with it, e.g., when using a database that is accessed from multiple hosts via a network share. When this setting is disabled, FTL will use SQLite3's default journal mode (rollback journal in DELETE mode).

This PR was initially triggered by this Discourse thread, however, by now I think that WAL wasn't the issue here but specific user configuration I had not noticed before in the debug log.

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:

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.
I have commented my proposed changes within the code.
I am willing to help maintain this change if there are issues with it later.
It is compatible with the EUPL 1.2 license
I have squashed any insignificant commits. (git rebase)

Checklist:

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

pi-hole / FTL

Make WAL mode for pihole-FTL.db optional #1868

What does this implement/fix?

Checklist: