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. Test Plan: Run tests: ./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