R1:c1eeacd8500b

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…