perf(filer): pre-size chunk stream reader buffer to view size

The chunk size is known up front, so grow the buffer once instead of
letting bytes.Buffer double as the streamed pieces arrive (which
transiently overshoots to ~2x per reader).

weed/filer/stream.go

@@ -484,6 +484,8 @@ func (c *ChunkStreamReader) fetchChunkToBuffer(chunkView *ChunkView) error {
 		return err
 	}
 	var buffer bytes.Buffer
+	// pre-size to the known chunk size; avoids bytes.Buffer's doubling regrowth
+	buffer.Grow(int(chunkView.ViewSize))
 	var shouldRetry bool
 	jwt := JwtForVolumeServer(chunkView.FileId)
 	for _, urlString := range urlStrings {