[Varown] Cascading deletes

[JustusAdam]

Deletions do not cascade beyond the the pointed-to data.

We need to figure out if it should.

Handling OWNS columns in delete does not append the records that are removed in the step to the ones used to update the flow. This should be correct, because it is only supposed to delete copies in user shards. However one weird corner case this creates is that if the last "connection" to such an owned item is deleted, the element remains in the dataflow, but is no longer actually stored in the database. This should go away once we figure out the precise handling of such relations with respect to deletes.

[KinanBab]

I want to revisit this whole cascading deletes discussion after we finish the indices clean up and post submission clean (so ~ June)

I think we should also think about cascading deletes / integrity errors beyond variable ownership and for cross-shard scenarios.