lfs: default to not using workers for upload/download
I ran into truncated uploads with this defaulting to on. Wojciech Lis diagnosed
it as creating keepalive connections prior to forking, and illegally
multiplexing the same connection. [1] I didn't notice a problem with the couple
of downloads I tried, but disabled both for simplicity and safety.
[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-January/109916.html