Consider v2 attachments on conflicting revisions when removing

Currently V2 attachment removal only considers attachments on the previous active revision and the incoming revision.  This covers CBL 2.x cases (no conflicts).  

When running with allow_conflicts=true, SG should be checking for v2 attachments on all non-tombstoned leaf revisions.

To do this efficiently (i.e. avoid retrieving and unmarshalling all non-tombstoned leaf revisions on every update), we should consider adding a flag to non-active leaf revisions indicating whether they include v2 attachments.