Cursor dropping doesn't respect the purge-seqno

The following sequence demonstrates the issue (this has been performed via a unit-test)

• Create DCP and stream from vb0
• store into vb0 k1,k2,k3,k4 and k5
• stream all items, client knows about k1,k2,k3,k4 and k5
• delete k3 and k4 (we have to delete a minimum of 2 items because compaction never purges a delete if its the last seqno)
• run compaction - k3 tombstone is purged
• stream is slow... so we drop its cursor and go to backfill mode
• backfill finds delete(k4)
• client streams all items available and gets delete(k4)

Database has k1,k2,k5
Client has k1,k2,k3,k5

The cursor drop backfill was allowed to go beyond the purge-seqno, a normal stream request with backfill would be a rollback situation.