We have code that tracks when a page has a lot of deleted entries associated with it, and tries to force evict that page. We do that because it's inefficient to skip many deleted entries every time a page is looked at.

For historical reasons related to the lookaside file we've excluded tombstones (deletes) that have a WT_TXN_NONE transaction ID. See WT-3768 for details.

We should revisit that decision now that we don't have the lookaside file any longer.