Move "Wait for Previous Commits to Build" out of prototype
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.
Test Plan: Added a new build step, saw this as an option in the "Flow Control" section.
Differential Revision: https://secure.phabricator.com/D21432