De-functorize MultiWorkerLwt
De-functorize MultiWorkerLwt

Summary: `MultiWorker.CallFunctor` is basically empty at the moment, so it's pretty easy to de-functorize `MultiWorkerLwt`

Reviewed By: samwgoldman

Differential Revision: D18983804

fbshipit-source-id: db26db44374c2ab3ceb1f69adff65dabc91b2fab

mroch/flow,mroch/flow,samwgoldman/flow,facebook/flow,samwgoldman/flow,mroch/flow,mroch/flow,facebook/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,samwgoldman/flow,facebook/flow,samwgoldman/flow,facebook/flow,facebook/flow