(ITS#8900) contextCSN issue for export if last op was to delete an entry

Full_Name: Quanah Gibson-Mount Version: 2.4.46 OS: N/A URL: ftp://ftp.openldap.org/incoming/ Submission from: (NULL) (47.208.148.239)

When making an export for backup using slapcat there could be issues on restore if the last operation logged was for a delete entry operation, as the contextCSN would then point to an entry that no longer exists. If that backup was then used at restoration time in an replicated environment and the master gets a write op before any replicas are online, the replicas will fallback to REFRESH mode since they can't find entry the contextCSN points to.