[bug] parallel calling fGetObject() using local cache will cause unexpected error

[fishbar]

the local tmp file for speed up download is dangeruos！ when the fGetObject() is parallel called with same arguments, the cache file will be polluted， the cache file will be filled with origin file’s fragment in chaos order（deps on each caller network loading speed）

node js example:

so the local cache shoud be banned by default， for Advanced User using option to enable it