Today we carry over hard deletes based on the SegmentReaders liveDocs.
This is not correct if soft-deletes are used especially with rentention
policies. If a soft delete is added while a segment is merged the document
might end up hard deleted in the target segment. This isn't necessarily a
correctness issue but causes unnecessary writes of hard-deletes. The biggest
issue here is that we assert that previously deleted documents are still deleted
in the live-docs we apply and that might be violated by the retention policy.