database: various optimizations for SQLite and PostgreSQL

doc/admin.rst

@@ -287,6 +287,16 @@ can be changed by this setting
   # keep messages forever, the default
   archive_expires_after=0
 
+sqlite_journal_mode
+~~~~~~~~~~~~~~~~~~~
+
+Set the `journal_mode https://sqlite.org/pragma.html#pragma_journal_mode`_ to
+be used when connecting to an SQLite database. This option can be one of
+`DELETE`, `TRUNCATE`, `PERSIST`, `MEMORY`, `WAL`, `OFF`, or `none` to not set
+the journal_mode at all.
+
+By default this option is set to `WAL` to use `Write-Ahead Logging
+https://sqlite.org/wal.html`_.
 
 TLS configuration
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