backupccl: use rdi.LazyIterator to avoid loading all ranges at once

Release note (enterprise change): BACKUPs now load range information as they need it to avoid a spike in metadata lookups when backups begin.
Epic: none.

pkg/ccl/backupccl/backup_processor.go

@@ -333,7 +333,8 @@ func runBackupProcessor(
 			remainingSpan := fullSpan
 
 			if rangeSizedSpans {
-				rdi, err := flowCtx.Cfg.ExecutorConfig.(*sql.ExecutorConfig).RangeDescIteratorFactory.NewIterator(ctx, fullSpan)
+				const pageSize = 100
+				rdi, err := flowCtx.Cfg.ExecutorConfig.(*sql.ExecutorConfig).RangeDescIteratorFactory.NewLazyIterator(ctx, fullSpan, pageSize)
 				if err != nil {
 					return err
 				}
@@ -347,6 +348,9 @@ func runBackupProcessor(
 					requestSpans = append(requestSpans, spanAndTime{span: subspan, start: start, end: end})
 					remainingSpan.Key = subspan.EndKey
 				}
+				if err := rdi.Error(); err != nil {
+					return err
+				}
 			}
 
 			if remainingSpan.Valid() {