New splitting strategy for providers

Old format:
 - **latest**: until 10 hours before 1st day of current month.
 - **active**: 10-60 hours before 1st day of current month.
 - **stale**: 60-180 hours before 1st day of current month.
 - **archived**: more than 180 hours before 1st day of current month.

New format:
 - **latest**: until monday last week.
 - **YYYY-MM**: month block (03, 06, 09, 12), excluding what is in latest.
 - **YYYY**: year, excluding current
 - **archived**: Everything older than 2012

Rationale:

 * Split into smaller groups, resulting in faster download. This becomes a problem when Packagist is overloaded and/or the user has a slow connection - Composer times out.
 * Greatly reduces the changes to old providers. With the previous strategy, packages were switching constantly, causing cascading changes to all providers.

There is still the problem that the dumper relies on file modification time, it would be best to switch to actual package modification date.