A collection with 100% items purged sends a snapshot marker but not the end

Summary

• Scenario results in DCP sending a snapshot marker and no data.
• Indexer likely to become stuck (similar issue to MB-37209)
• Note that buckets (and anything prior collections) avoid this because we will never purge the item @ high-seqno, thus DCP would always find a tombstone to represent the end of the snapshot.
• Issue only affects collection streams and as of now we would purge the item @ collection high-seqno.

Steps to reproduce.

1. create 1 nodes
2. Create a number of collections and add documents to all collections (i used enough so all vbuckets had data)
   1. Do this in sequence though. E.g if you have collections A, B and C. Add all documents to A then all to B and finally all to C.
3. Delete documents from either collection B (or A), can be done by giving a small expiry at the write phase and forcing compaction.
4. Purge all tombstones - I had a developer build so was able to trick the purger into thinking everything was valid for purging regardless of the 3 day purge interval.

Now that the node is set-up, we can observe that collection B has no items, but a DCP stream which requests collection B only will yield the following.

• DCP snapshot, flags=disk, start 0, end collection high-seqno (or high-seqno if MB-32231 is not fixed)
• DCP system-event (collection B created @ seqno x)
• No more data, stream will not send anything for the end seqno the marker mentioned

Ideas to resolve:

• Make the purger collection aware, it should never purge the collection's item at its high-seqno
• More DCP messages, we could use DCP_SEQNO_ADVANCED in place of the hidden missing end item?