Add RunningStats helper

In the simulation server we were calling get_engine_stats() each tick to save some stats for the timeseries. However get_engine_stats has several sums/mins that end up being n^2 when if we run get_engine_stats() every tick and bog down the simulation.

Add a RunningStats class that can be used for keeping running totals etc.

Remove the prior "fast" toggle I added to get_engine_stats as its not really relevant anymore.