filer: add option to cache small files to filer store

weed/server/filer_server.go

@@ -55,6 +55,7 @@ type FilerOption struct {
 	Port               uint32
 	recursiveDelete    bool
 	Cipher             bool
+	CacheToFilerLimit  int64
 	Filers             []string
 }
 

weed/server/filer_server_handlers_write_autochunk.go

@@ -236,7 +236,7 @@ func (fs *FilerServer) uploadReaderToChunks(w http.ResponseWriter, r *http.Reque
 			break
 		}
 	}
-	if chunkOffset < 2048 {
+	if chunkOffset < fs.option.CacheToFilerLimit {
 		smallContent = content
 	}
 	return fileChunks, md5Hash, chunkOffset, nil, smallContent