Update db-bench

[vweevers]

With this, you can plot multiple runs. For example, repeated runs to see if the benchmark is stable:

./bench/db-bench-plot.sh master-*.csv

Or comparing runs from branches (for this plot I rebased the napi branch on top of this PR):

./bench/db-bench-plot.sh master.csv napi.csv

(napi is slightly faster on Windows too!)

[ralphtheninja]

Awesome!