Update misleading statistics titles

### Problem

@fabiog1901 recently expressed concern over the accuracy of the statistics calculator.
> ... these are the stats output from the python workload movr: don't these stats look wrong?
> ```
> transaction name time(total) ops(total) ops ops/second p50(ms) p90(ms) p95(ms) max(ms)
> ------------------ ------------- ------------ ----- ------------ --------- --------- --------- ---------
> add vehicle 136 90 7 0.0515746 15.89 20.83 23.48 26.12
> apply promo code 136 172 19 0.139987 23.53 34.08 37.68 39.52
> end ride 136 399 37 0.272606 28.7 36.69 37.68 53.11
> get vehicles 136 28450 3307 24.3646 16.92 22.92 27.38 107.2
> log ride location 136 15550 1623 11.9574 15.21 18.71 20.82 57.89
> new promo code 136 48 4 0.0294694 15.75 18.39 18.93 19.47
> new user 136 397 41 0.30206 15.54 18.32 18.74 57.73
> start ride 136 449 55 0.405197 28.86 41.32 42.93 50.45
> ```
> Take the "get vehicles" row: in 136s the workload has processed 28450, so 28450/136= 209 ops/second, not 24. 24 is the result of the partial ops 3307/136, which does not make sense calculating. Rather, it should be ops/current_period_duration, that is, 3307/15s = 220

I think these column labels are just misleading. The `ops(total)` column label should be a cumulative total for the lifetime of the generator. It follows that `time(total)` should also be a cumulative time elapsed for the lifetime of the generator, but it appears that it is, in fact, just the lifetime of the stats calculator (what Fabio called `current_period_duration`).

`ops/second` is roughly `ops`/`time(total)` (https://github.com/cockroachdb/movr/blob/master/movr_stats.py#L45). The reason it is so imprecise appears to be because the time measurement displayed for `time(total)` is rounded to the nearest integer.

### Suggested resolution

Update the stats table titles to make the time statistic title less confusing:
https://github.com/cockroachdb/movr/blob/master/movr_stats.py#L54

We could also potentially update the time measurement to be a little more precise.