Hard dependency on threads

Hi!

The hard dependency on threads is problematic for any code that builds for js_of_ocaml. js_of_ocaml supports events but not threads so any code that pulls a dependency from the lock free package set, for instance saturn_lock_free gets a hard dependency on threads and cannot be run with js_of_ocaml

See: https://github.com/ocsigen/js_of_ocaml/issues/1569 for one such example.