Refactor ApplyBuckets Work

[SirTyson]

Currently, ApplyBuckets work reasons at the BucketLevel instead of at the per-bucket level. Especially after the BucketListDB apply optimization, this means that there's some tricky logic required to keep our current place in the BucketList and make sure our iteration order is correct. We should refactor this to apply per-bucket instead of per-level to simplify the logic.