This change should have no impact on behavior.
SQLite supports [several different checkpoint modes][0]. We always use
the "TRUNCATE" mode, so we can remove the code that handles different
ones.
We can also stop logging checkpoint results that are [always 0 in
"TRUNCATE" mode][1].
We can also use GRDB's WAL checkpoint method instead of calling SQLite's
API directly.
[0]: https://www.sqlite.org/c3ref/c_checkpoint_full.html
[1]: https://sqlite.org/c3ref/wal_checkpoint_v2.html
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