Added workers in lfs.
I had to remove the fine progress tracking because between processes in *nix and threads in windows (diffs will appear soon) the tracking of 1MB progress is quite tricky.
With our network tracking progress per file is way enough to see things moving.
This change gives close to 50% speedup on hg sparse --enable-profile when prefetch is run. My current understanding is that prefetch is ran when profile is enabled for the first time.