Stepback bisection

The stepback process right now steps back across versions linearly. This can lead to stepbacks taking a very long time to find a passing version depending on how long ago the commit that caused the task to begin failing was introduced. 

This ticket is to make the stepback step back and search for a passing version in a way similar to git bisect.

The ultimately goal here is that if we have a server task running weekly, and it fails, we should be able to bisect to the offending commit (or give up) within 10 or fewer steps. Assuming approximately 160 commits a week, we should be able to get to a offending commit in 7 steps. We think this will enable issue detection and possibly revert within 12 hours of a task failing. 

Logging to understand when git bisect concludes a successful search vs unsuccessful would be really valuable.