mds_kill_export_at:
1: After moving to STATE_EXPORTING
2: After sending MExportDirDiscover
-3: After receiving MExportDirDiscoverAck and auth_unpin'ing.
+3: After receiving MExportDirDiscoverAck and auth_unpinning.
4: After sending MExportDirPrep
5: After receiving MExportDirPrepAck
6: After sending out MExportDirNotify to all replicas