Timing history truncation/compaction

[jerluc]

Currently, whenever sw records timing statistics, it just keeps accumulating into the history.json file. While this provides the most accurate aggregate statistics, this also means that the history.json file size is unbounded and may eat away at disk space and degrade performance over time.

We should figure out how to either:

1. Truncate timing histories such that only the last X timings are persisted, OR
2. Figure out a way to compact the histories together in a way that leaves the aggregates relatively in tact (e.g. a moving average vs. mean)