Fix directory timestamps

[rossica]

The current file transfer model doesn't correctly set the timestamps on directories. What happens is the folder is created and the timestamp is set, but then files and directories are created under it, and those change the timestamp on the directory to the current day and time.
The solution here probably requires refactoring the way directory timestamps are set such that it happens AFTER the last file/directory is written to that directory.
Maybe have a std::stack where directories are added as they're received, and once all files have been received/written, pop the directories off the stack and correct the timestamps?