In the previous implementation, clone task will continue download files
even if some error happened. This may cause unexpected problem. This
Change List refactor it to that when error happends, clone task will
fail total and try to clone from another remote source.
Besides above change, I call FileUtils::remove_all and create_dir
instead of boost one, which may cause exception. What's more
AgentMasterClient is replaced with ThriftRpcHelper, by this change
conncection can be reused.