Fix TTL pruning logic

[jazg]

There are currently two issues with the pruning logic.

1. Data is divided into slots (current time / length of interval) which represent discrete time intervals. These slots are dropped when their distance is >= 1 from the current time slot. This means if data is inserted at the start of a slot and at the end, they will both be pruned when the next slot is created.

2. Inserting data with the same key does not update the time at which the data will be pruned. For example (assuming we have one hour prune intervals), if the user inserts data at 3pm and updates the data at 3:30pm, the pruning still occurs at 4pm since the key has not changed.

This PR does two things to fix these issues.

1. Increases the distance from 1 to 2. So (assuming the prune interval is one hour) rather than pruning data that is at most 1 hour old, it is pruned after at least 1 hour and at most 2 hours.

2. Removes the key from the previous slot when the data is updated.

[loongy]

LGTM. I have left one question and one comment, but nothing that would prevent this from being merged.