Are parameters `rolling` and `changed` optional when implement an external session store?
Excuse me. I am writing an external (redis) session store for koa-session, but I miss problem: 

```
- get(key, maxAge, { rolling }): get session object by key
- set(key, sess, maxAge, { rolling, changed }): set session object for key, with a maxAge (in ms)
- destroy(key): destroy session for key

```

I don't understand the meaning about parameters `rolling` and `changed`.  Are they optional ?  Can I just implement like this?:

- get(key, maxAge)
- set(key, sess, maxAge)
- destroy(key)

