Add support for storing a list of expired packages in the meta

[arrowd]

This is a draft PR that I wanted to show off before proceeding further.

• I generalized group_open_schema() into open_schema().
• group_load() and expired_load() may be generalized too.
• Add a config option that would remove expired packages during pkg upgrade? Or maybe pkg autoremove?
• What will actually pass -e to the pkg repo call? Changes are required on the Poudriere side?
• Bikeshedding? Should this be called "expired_packages" or somehow else?