StatusStorage::SQLite: set foreign key constraint to statuses.timeline_id. If we can use the constraint, we should definitely use it.
StatusStorage::SQLite: NOT NULL constraint to all columns. I think NULL is almost always evil.
StatusStorage::SQLite, Memory: set default max_statuses_num to 2000. 4000 is too many, I guess.
Add benchmark/vacuum_time.pl to measure VACUUM times of StatusStorage::SQLite.