`DELETE_SUBJECT` records - what are they used for?

[dmariassy]

Hi,

We have tried to do a mass clean up of our SR instance, and deleted a ton of subjects with ?permanent=true. These requests produced the expected tombstones, and will get compacted away.

However, the non-permanent deletes that needed to precede the ?permanent=true requests have generated a lot of DELETE_SUBJECT messages. These have the same cardinality as the input subject versions, and don't have corresponding tombstones.

As a result, it looks like we will be left with the exact same number of messages in our _schemas topic as when we started the clean up exercise.

Is this intentional? Are we missing something?

Originally posted by @dmariassy in #1429