Drop garbage containers bucket

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we have more buckets in metabase than we need.

Describe the solution you'd like

Garbage containers bucket can be replaced by a marker (special prefix) in respective meta bucket. If there is no bucket, there is nothing to delete anyway. This would simplify various checks since we always have meta bucket cursor somewhere around.

Describe alternatives you've considered

Things work as is, but suboptimal.

Additional context

6d4beeb

[roman-khimov]

Optimize some checks while here. We don't need to check it per-object, we only need to check it per-container.