(wangle) Return a Later from Future::via

(wangle) Return a Later from Future::via

Summary: Stroke of brilliance, Hannes.

Test Plan:
unit tests, including a new one
Looked through `fbgs 'via('` and all the extant `via`s are attached to `Later`s already so it shouldn't break anything. But check contbuild before commit.

Reviewed By: hannesr@fb.com

Subscribers: net-systems@, fugalh, exa

FB internal diff: D1406976

Tasks: 4480567