Filter out 'stale' requests

Requests that are stale are non-closed old time slot requests. These
showed up because the checkForUpcomingRequestsSlot function only filtered
upwards of the starting time of a slot.

By adding a lower bound for the filter on slot.opensAt, requests from longer
than 1 day ago should no longer show up.

Additional changes:
- A log message telling the time is removed as it did not any necessary information.
- An unused field for a datetime format is removed.

Signed-off-by: Chris Lemaire <c.lemaire@student.tudelft.nl>