Opaque key naming the request this delta belongs to, when known.
Keys each correction to the request it applies to. A late revision then
cannot subtract spend that belongs to other requests. None when the
message carried no usable ID, which is also when the ledger refuses to key
the request at all.
Built by _provisional_bucket_key, so it separates retry attempts exactly
as the ledger does. Treat it as a bucket key, not as a provider message ID.
