filer: server side copying (#7121)

* copy

* address comments

* remove unused functions, reuse http clients

* address hardlink, checking existing directory

* destination is directory

* check for the key's existence in the map first before accessing its members

* address comments

* deep copy remote entry

* address comments

* copying chunks in parallel

* handle manifest chunks

* address comments

* errgroup

* there could be large chunks

* address comments

* address comments

weed/server/filer_server_handlers_write.go

@@ -116,6 +116,8 @@ func (fs *FilerServer) PostHandler(w http.ResponseWriter, r *http.Request, conte
 
 	if query.Has("mv.from") {
 		fs.move(ctx, w, r, so)
+	} else if query.Has("cp.from") {
+		fs.copy(ctx, w, r, so)
 	} else {
 		fs.autoChunk(ctx, w, r, contentLength, so)
 	}