Add SQLite WAL and busy_timeout

SQLite has rather unfortunate defaults for concurrent accesses, leading to database locked errors. https://github.com/matrix-org/dendrite/pull/1290 appears to try to fix some of this, but doesn't appear to cover a concurrent read+write and the errors still happened in large quantities when I joined the dendrite room.

This PR enables WAL for concurrent read+write and much better performance. It also sets busy_timeout to 10 seconds to not immediately fail operations while the database is locked, providing a DB level solution to PR 1290's objective with additional coverage.

I chose this particular location to add the two statements as they should apply to every SQLite DB connection.

From a quick test it looks as if the db locked errors are gone with this change.

internal/sqlutil/sqlutil.go

@@ -46,6 +46,9 @@ func Open(dbProperties *config.DatabaseOptions, writer Writer) (*sql.DB, error)
 		db.SetMaxOpenConns(dbProperties.MaxOpenConns())
 		db.SetMaxIdleConns(dbProperties.MaxIdleConns())
 		db.SetConnMaxLifetime(dbProperties.ConnMaxLifetime())
+	} else {
+		db.Exec("PRAGMA busy_timeout = 10000;")
+		db.Exec("PRAGMA journal_mode=WAL;")
 	}
 	return db, nil
 }