cp file can work

1. consolidate to filer_pb.FileChunk
2. dir add file, mkdir
3. file flush, write

updates having issue

weed/filer2/filechunks.go

@@ -1,6 +1,8 @@
 package filer2
 
-type Chunks []FileChunk
+import "github.com/chrislusf/seaweedfs/weed/pb/filer_pb"
+
+type Chunks []*filer_pb.FileChunk
 
 func (chunks Chunks) TotalSize() (size uint64) {
 	for _, c := range chunks {