Enable WAL for SQLite (#13897)

Signed-off-by: Lorenzo Manacorda <lorenzo@mailbox.org>

changelog.d/13897.feature

+Enable Write-Ahead Logging for SQLite installs. Contributed by [asymmetric](https://github.com/asymmetric).

synapse/storage/engines/sqlite.py

@@ -88,6 +88,10 @@ class Sqlite3Engine(BaseDatabaseEngine[sqlite3.Connection, sqlite3.Cursor]):
 
         db_conn.create_function("rank", 1, _rank)
         db_conn.execute("PRAGMA foreign_keys = ON;")
+
+        # Enable WAL.
+        # see https://www.sqlite.org/wal.html
+        db_conn.execute("PRAGMA journal_mode = WAL;")
         db_conn.commit()
 
     def is_deadlock(self, error: Exception) -> bool: