Use multiple decay functions to boost recently created and
(more importantly) recently modified documents.
This actually works in reverse, so objects older than a certain
offset get a negative boost based on a decay function.
age > 1d, reduce score by 20% each 10 days.
age > 10 days, reduce score by 20% each 90 days
age > 90 days, reduce score by 20% each 180 days