lfs: using workers in lfs prefetch
This significantly speeds up lfs prefetch. With fast network we are
seeing ~50% improvement of overall prefetch times
Because of worker's API in posix we do lose finegrained progress update and only
see progress when a file finished downloading.
./run-tests.py -l test-lfs*
Ran 4 tests, 0 skipped, 0 failed.
Run commands resulting in lfs prefetch e.g. hg sparse --enable-profile
Differential Revision: https://phab.mercurial-scm.org/D1568