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_pattern.go

@@ -4,19 +4,17 @@ type WriterPattern struct {
 	isStreaming     bool
 	lastWriteOffset int64
 	chunkSize       int64
-	fileName        string
 }
 
 // For streaming write: only cache the first chunk
 // For random write: fall back to temp file approach
 // writes can only change from streaming mode to non-streaming mode
 
-func NewWriterPattern(fileName string, chunkSize int64) *WriterPattern {
+func NewWriterPattern(chunkSize int64) *WriterPattern {
 	return &WriterPattern{
 		isStreaming:     true,
 		lastWriteOffset: -1,
 		chunkSize:       chunkSize,
-		fileName:        fileName,
 	}
 }
 
@@ -39,3 +37,8 @@ func (rp *WriterPattern) IsStreamingMode() bool {
 func (rp *WriterPattern) IsRandomMode() bool {
 	return !rp.isStreaming
 }
+
+func (rp *WriterPattern) Reset() {
+	rp.isStreaming = true
+	rp.lastWriteOffset = -1
+}