Stats collection causes unbounded memory growth, slowing RPS over time

## Motivations
- `oha` memory usage grows about 2 MB every couple of seconds under normal usage conditions at 11k RPS
- After a few minutes, `oha` slows down to only 8-9k RPS
- Restarting `oha`, but not the target of the test, restores the 11k RPS rate
- It appears that the increasing memory usage for stats may be causing the reduction in performance over time
- While the requests *should* not be slowed down by the stats collection, they are going to compete for CPU with the stats processing and the stats processing may be using more CPU as the collection of stats grows larger, which could explain why the RPS rate starts to drop

## Slowing RPS Over Time

## Restarting `oha` but not Target Restores RPS