Observations database clearing improvements

[samsymons]

Task/Issue URL: https://app.asana.com/0/0/1201945748431037/f
Tech Design URL:
CC: @ladamski

Description:

This PR updates the logic used to clear Apple's observations.db. There are three changes:

1. Before clearing, a checkpoint call is made to ensure that the data in the wal file is moved into the main database. The observations database uses Write-Ahead Logging, hence the need for this call.
2. After clearing, a VACUUM call is made to tell the database to clean up after itself.
3. Despite these first two points, sometimes domains would still be present when running strings over the database, so the delete call is run more time - it's unclear to me why, but this final step triggers SQLite to fully clean up after itself in my testing.

For further context: this database is used by Intelligent Tracking Protection, and stores a list of domains used by that feature. We have no official way to clear or disable it, hence the direct SQL usage.

Steps to test this PR:

1. Open ~/Library/WebKit/com.duckduckgo.macos.browser.debug/WebsiteData/ResourceLoadStatistics in your terminal
2. Run strings observations.db-wal observations.db | grep ".com", which will show you a list of domains that you've visited - gross!
3. Run the app from this branch and then use the Fire button2.
4. Run strings observations.db-wal observations.db | grep ".com again, you should get no results back - if you do, then this PR should not be approved and further investigation is needed
5. Check the size of the files in the directory with ls -la - it's expected that the observations.db-wal file is totally empty. This isn't a hard requirement of the PR (we only care if any data is left over), but is expected due to the checkpoint call
6. Lastly, do some browsing in the app and then run strings observations.db-wal observations.db | grep ".com" one last time, it should once again show you some of the domains you've visited. This is unfortunately expected, and unavoidable, they'll get cleared again the next time you use the Fire button.

Testing checklist:

• Test with Release configuration
• Test proper deallocation of tabs
• Make sure committed submodule changes are desired

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Internal references:

Software Engineering Expectations
Technical Design Template
When ready for review, remember to post the PR in MM

[tomasstrba]

LGTM! 👍 💯 I did not test anything in this PR but Lucas did! 😆