Update caching strategy

[leonardehrenfried]

I noticed that the OSM extract for NE Italy was never updated in the cache and the daily updates didn't reach back far enough.

Therefore I'm updating the cache strategy as follows:

• at the beginning of the month a new extract is downloaded from Geofabrik
• daily updates are applied throughout the month
• at the next month a fresh extract is downloaded

[leonardehrenfried]

cc @dulvui