Let private bookings expire after some time

[dahaic]

Private bookings are there for safe start of galaxy for friends or somehow coordinated group. Given the nature of outerspace require players to log in at least once per two days (otherwise the handicap is too big and it's not worth the time), there is no point in letting the booking to exist for more than a week.

Proposing to expire / remove private bookings that are older than one week.