Add a flag to non-active leaf revisions indicating whether they include v2 attachments

Currently, v2 attachment removal only considers attachments on the previous active revision and the incoming revision. This covers CBL 2.x cases (no conflicts).  

However, SG should be checking for v2 attachments on all non-tombstoned leaf revisions when running in allowed conflict mode. It requires retrieving and unmarshalling all non-tombstoned leaf revisions on every update. This approach is not efficient and we should consider adding a flag to non-active leaf revisions indicating whether they include v2 attachments.