Collections: OSO backfill for a yet to be flushed collection send OSO start/end

Logging this issue which covers a case where DCP probably sends messages when we could get away with sending less.

If we have a collection created, but have yet to flush the collection to disk and an OSO stream-request for that collection is created KV will send:

• DCP OSO start
• DCP OSO end

With nothing inbetween, we would then switch to in-memory streaming and pickup the unflushed DCP system event (which triggers DCP Create Collection).

If the stream were a disk-only request, we send close the stream after the DCP OSO end.

I have some tests to push which highlight the issue