[Ephemeral] Backfill may miss purged deletes due to race at create

This bug affects Ephemeral buckets only.

When a backfill is created, it should check that the purgeSeqno < startSeqno, to ensure that the client will not miss deletes.

Ephemeral buckets store changes in a linked list (the sequence list). Ranges of changes from this list can be locked (a read range) when they are needed for backfills.

In the backfill initialisation code in DCPBackfillMemoryBuffered::create, we create a read range (makeRangeIterator). This prevents tombstone purging. Then we check purgeSeqno < startSeqno.

When we do, we read the purgeSeqno from the VBucket object (VBucket::purgeSeqno), not from the LinkedList::getHighestPurgedDeletedSeqno and this is where the race can happen.

When tombstone purging occurs in EphemeralVBucket::purgeStaleItems, we call LinkedList::purgeTombstones(), which operates under the sequence list lock, but the update of the VBucket::purgeSeqno is not synchronised with that.

The backfill create can see the old purgeSeqno in this instance, and allow a backfill to be created which will miss deletes.