Introduce Sqlite WAL journal mode

[lainedfles]

• Add Write-Ahead Logging option for file mode database
• Disable by default
• Migrate journal_mode and related PRAGMA statement handling from _create_tables() to new functions: get_journal_mode()/set_journal_mode()

This yields a significant reduction of constant IO writes on my systems when db file mode is enabled.

Note that PRAGMA synchronous = NORMAL doesn't appear to activate properly. This problem occurs on master branch builds as well. Manual inspection of the db shows that synchronous always = FULL unless manually set directly but this warrants additional discussion elsewhere.

[gustavo-iniguez-goya]

Thank you @lainedfles !

I experienced a lot of DB corruptions using WAL and other modes when I added the option to save events to disk.
That's why I ended up using previous options.

But things has changed a lot since then, and allowing to configure it is great :)

[lainedfles]

@gustavo-iniguez-goya I wondered about the implementation based on this comment:

https://github.com/evilsocket/opensnitch/blob/6556eed1aeb124f22d8ef503c4ba960f38918654/ui/opensnitch/database/__init__.py#L90C9-L90C9

So far I've not experienced corruption 🤞🏻. I've created #1014 to track the failure to set NORMAL mode but I wonder if this option warrants an additional GUI configuration for cases where corruption is a problem.