perf(storagenode): estimate the number of batchlets

This PR estimates the number of batchlets more accurately. Previously, the number of batchlets is
estimated by dividing the size of batches by the smallest batchlet. It is now divided by the right
size of batchlet.

internal/storagenode/logstream/append.go

@@ -42,9 +42,16 @@ func (lse *Executor) Append(ctx context.Context, dataBatch [][]byte) ([]snpb.App
 	startTime := time.Now()
 	var preparationDuration time.Duration
 	dataBatchLen := len(dataBatch)
+
+	_, batchletLen := batchlet.SelectLengthClass(dataBatchLen)
+	batchletCount := dataBatchLen / batchletLen
+	if dataBatchLen%batchletLen > 0 {
+		batchletCount++
+	}
+
 	apc := appendContext{
-		sts:  make([]*sequenceTask, 0, dataBatchLen/batchlet.LengthClasses[0]),
-		wwgs: make([]*writeWaitGroup, 0, dataBatchLen/batchlet.LengthClasses[0]),
+		sts:  make([]*sequenceTask, 0, batchletCount),
+		wwgs: make([]*writeWaitGroup, 0, batchletCount),
 		awgs: make([]*appendWaitGroup, 0, dataBatchLen),
 	}