directory timestamps modified behind user's back

[GoogleCodeExporter]

Due to the background operations (deduping and BG compression) being performed 
asynchronously, the directory timestamps are sometimes updated even though we 
don't want that.

Since the operations causing these timestamp updates are fast (link() for 
dedup, rename() for compression), we could lock the directory while doing them 
and write the old timestamps back again after completion. Problem: There is no 
mechanism in place for locking entire directories.

Without locking, we would risk losing legitimate updates caused by 
modifications to entries in the directory that happen while we do our thing.

Original issue reported on code.google.com by ulrich.h...@gmail.com on 8 Nov 2011 at 12:04