(wangle) s/continuation/callback

(wangle) s/continuation/callback

Summary: The word "continuation" is too ambiguous. Prefer callback (except where we are actually talking about CSP continuations in the README).

Test Plan: Still builds. Nothing external is or should be calling `setContinuation`. But I'll wait for contbuild anyway.

Reviewed By: hannesr@fb.com

Subscribers: net-systems@, fugalh, exa

FB internal diff: D1406753

Tasks: 4480567