Add p95 to stats summaries

[mhofman]

As requested by @Tartuffo, this change refactors the stats summary generation and adds p95 percentiles to the values for which we generated averages.

I've also added a script which parses a perf.jsonl stream and rewrites the stats, regenerating the summaries. This helped me verify that the new summary logic didn't result in any real changes (just a few differences due to rounding).

for i in manual-*
do 
  echo $i
  diff \
    <(tail -q -n 1 $i/perf.jsonl | jq '.stats') \
    <(cat $i/perf.jsonl | ../runner/bin/regen-perf.js | tail -q -n 1 | jq '.stats')
done

I also fixed a bug where the stage stats summaries were not generated for a failing stage.

Related to: 5d4034a in #36

[Tartuffo]

Thanks, @mhofman !

[warner]

excellent