fix #177, add a lock file for n

before the version installed, touch a file named `n.lock` in version directory, and util the version has been installed/downloaded locally, remove the file.

Then for that checking function, just check the n.lock of the corresponding version directory is existed.
/cc @tjwebb
I'd rather use `rm -f` here. I don't want the user to be confused by any extraneous `rm`-related notifications, nor have to parse any errors if somehow `n.lock` doesn't exist here.