All changes:

* Improvement: Rather than randomly cleaning up expired sessions that will now happen at the `options.pruneSessionInterval` defined interval.
* Breaking change: Clients now need to close the session store to gracefully shut down their app as the pruning of sessions can't know when the rest of the app has stopped running and thus can't know when to stop pruning sessions if it itsn't told so explicitly through thew new `close()` method – or by deactivating the automatic pruning by settinging `options.pruneSessionInterval` to `false`. If automatic pruning is disabled the client needs to call `pruneSessions()` manually or otherwise ensure that old sessions are pruned.