Move "Wait for Previous Commits to Build" out of prototype

Summary:
Although I'm not entirely thrilled about doing flow control like this (as an actual action in a build plan), I believe this build step works correctly and there's no fancy replacement mechanism on the immediate horizon, and this didn't send us down a slippery slope of Turing-complete builds encoded without real structure or context. Just kick it out of prototype.

(Other approaches which might be better in the long run are things like "this is a top-level behavior on the build plan itself" and/or "build plans are written in a DSL, not a Javascript UI".)

Test Plan: Added a new build step, saw this as an option in the "Flow Control" section.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D21432