[MM-29666] compliance export skipped posts (#17445)

* add MessageExportCursor

* sort by PostUpdateAt and PostId

* re-run CI tests

* remove panic debugging line

Co-authored-by: Martin Kraft <martin@upspin.org>

store/opentracinglayer/opentracinglayer.go

@@ -2801,7 +2801,7 @@ func (s *OpenTracingLayerComplianceStore) GetAll(offset int, limit int) (model.C
 	return result, err
 }
 
-func (s *OpenTracingLayerComplianceStore) MessageExport(after int64, limit int) ([]*model.MessageExport, error) {
+func (s *OpenTracingLayerComplianceStore) MessageExport(cursor model.MessageExportCursor, limit int) ([]*model.MessageExport, model.MessageExportCursor, error) {
 	origCtx := s.Root.Store.Context()
 	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "ComplianceStore.MessageExport")
 	s.Root.Store.SetContext(newCtx)
@@ -2810,13 +2810,13 @@ func (s *OpenTracingLayerComplianceStore) MessageExport(after int64, limit int)
 	}()
 
 	defer span.Finish()
-	result, err := s.ComplianceStore.MessageExport(after, limit)
+	result, resultVar1, err := s.ComplianceStore.MessageExport(cursor, limit)
 	if err != nil {
 		span.LogFields(spanlog.Error(err))
 		ext.Error.Set(span, true)
 	}
 
-	return result, err
+	return result, resultVar1, err
 }
 
 func (s *OpenTracingLayerComplianceStore) Save(compliance *model.Compliance) (*model.Compliance, error) {