automatically select storage class

[sbrudenell]

for aws at least, it will make a lot of sense to select different storage classes for different backups. for example if our retention policy calls for a yearly backup, we know that backup shouldn't get deleted for a year, so we should store that backup as glacier deep archive.

this should be fairly straightforward since resolver's KeepMeta already exposes the intervals for which a backup should be kept.

for now this won't include migrating storage class of existing backups.

we should have naive support for aws, and include configuration to customize storage classes for different clouds.

we should be sure to have graceful recovery if we fail to delete a backup because of storage class rules, and if it makes sense, have an option to force deletion.