Optimize

Overall up to 25% reduction in garbage on cursor select workflows.

- Don't be so aggressive with selectLimit in pipelineRangeByScore
- Reduce allocations in pipelineRangeByScore (cursor)
- Reduce allocations in pipelineRange (non-cursor)
- Reduce allocations in unionDifference (minor)
- Route POST /debug to the pprof handler (long-time oops)
Pull fmt.Sprint out of pipelineRangeByScore inner loop
Fix TestCursorRetries test
Merge pull request #28 from soundcloud/optimize

Optimize