Feature/jsa/new handle threads

@gwprice 

notes:
* goal of optimization was to make GET /api/v1/threads faster.  load test results indicate this was definitely a success.  
* lt results summary: https://gist.github.com/jimabramson/a4e19eb9cc56941216f0
* much more refactoring possible.  i started down that road, then backed away, because it's a rabbit hole, and it's taken a long time to get here.  we have other upcoming stories for specific optimizations, and think we've paid down a reasonable amt of debt with the test coverage + performance improvements here.