Open opens the given file and returns it as a *DB.
The file is created if it doesn't already exist.
The database schema is created in the file if needed.
Callers should call Close when finished operating on the database.
Add adds a hash to db.
If it is already present, its last-access time is updated.
If db was opened with the Keep option,
entries with old last-access times are evicted.
Keep is an Option that sets the amount of time to keep a database entry.
By default, DB keeps all entries.
Using Keep(d) allows DB to evict entries whose last-access time is older than d.