add stream writer

this should improve streaming write performance, which is common in many cases, e.g., copying large files.

This is additional to improved random read write operations: 3e69d19380...19084d8791

weed/filesys/page_writer/chunked_stream_writer_test.go

@@ -0,0 +1,33 @@
+package page_writer
+
+import (
+	"github.com/stretchr/testify/assert"
+	"os"
+	"testing"
+)
+
+func TestWriteChunkedStream(t *testing.T) {
+	x := NewChunkedStreamWriter(20)
+	defer x.Reset()
+	y := NewChunkedFileWriter(os.TempDir(), 12)
+	defer y.Reset()
+
+	batchSize := 4
+	buf := make([]byte, batchSize)
+	for i := 0; i < 256; i++ {
+		for x := 0; x < batchSize; x++ {
+			buf[x] = byte(i)
+		}
+		x.WriteAt(buf, int64(i*batchSize))
+		y.WriteAt(buf, int64((255-i)*batchSize))
+	}
+
+	a := make([]byte, 1)
+	b := make([]byte, 1)
+	for i := 0; i < 256*batchSize; i++ {
+		x.ReadDataAt(a, int64(i))
+		y.ReadDataAt(b, int64(256*batchSize-1-i))
+		assert.Equal(t, a[0], b[0], "same read")
+	}
+
+}