Investigate adopting orjson/orjsonl library for at least some json operations to improve performance

Given the amount of JSON we're throwing around, we may want to look at switching over to [`orjson`](https://github.com/ijl/orjson) and [`orjsonl`](https://github.com/umarbutler/orjsonl) for json encode/decode, particularly for our more intensive operations (BatchPoster, Legacy ID map decode, etc.).