locking in zipfs demo

[Joel Uckelman]

Thus spake Alan Bateman:
> 
> Joel - another approach to consider would be simply change 
> ZipFilePath.{begin,end} to invoke fileSystem.{begin,end}. It would avoid 
> changing each usage (I don't have a strong opinion either way).
> 

I think calling the lock via an FS method makes it clearer that it's an
FS-level lock. Calling it via a Path method was the thing which made me
wonder whether it was correct in the first place.
 
-- 
J.