Summary:
`GHC.Conc` provides almost everything we need.

Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>

Test Plan:
    make test TEST=T9379

Reviewers: austin, simonmar

Reviewed By: austin, simonmar

Subscribers: phaskell, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D152