block/compact: rework consistency check, make writers only write

[GiedriusS]

• It's weird that on upload errors, we try to clean everything and only then write again. It's an extra operation we don't need since whether a block exists or not hinges on the existence of meta.json. We don't need to delete old, same objects before trying to upload them again.
• Consequently, we need to always use the upload time, not block creation time when checking for consistency or when deleting partially uploaded blocks. Directories as such don't exist in object storages, it's a client-side "illusion", so we need to iterate through the partial block's directory to fetch the last modified date.
• Finally, it's a good principle in general to make writers only write - this makes it easier to reason about how the whole system works!

Based on old #2883, #2585