perf(compression): Use gzip with BestSpeed in export and backup (#7643)

The export and backup code were using BestCompression and Default
gzip compression levels respectively. We see a 4x speed improvement when
switching from BestCompression to BestSpeed compression level.

(cherry picked from commit 5676462)

worker/backup_processor.go

@@ -139,9 +139,12 @@ func (pr *BackupProcessor) WriteBackup(ctx context.Context) (*pb.BackupResponse,
 
 	newhandler, err := enc.GetWriter(x.WorkerConfig.EncryptionKey, handler)
 	if err != nil {
-		return &response, err
+		return &response, errors.Wrap(err, "failed to get encWriter")
+	}
+	gzWriter, err := gzip.NewWriterLevel(newhandler, gzip.BestSpeed)
+	if err != nil {
+		return &response, errors.Wrap(err, "failed to create new gzip writer")
 	}
-	gzWriter := gzip.NewWriter(newhandler)
 
 	stream := pr.DB.NewStreamAt(pr.Request.ReadTs)
 	stream.LogPrefix = "Dgraph.Backup"

worker/export.go

@@ -378,7 +378,7 @@ func (writer *fileWriter) open(fpath string) error {
 	if err != nil {
 		return err
 	}
-	writer.gw, err = gzip.NewWriterLevel(w, gzip.BestCompression)
+	writer.gw, err = gzip.NewWriterLevel(w, gzip.BestSpeed)
 	return err
 }