Sentry-arroyo

Various fixes & improvements

- perf(collect): Add ParallelCollect step (41) by nikhars

Various fixes & improvements

- Increase log level (39) by fpacifici
- feat(perf) Add latency metrics to the messages coming from the commit log (38) by fpacifici

- Number of processes in the multi-process poll metric added. Its key is
`transform.processes`.

- Handle missing `orig_message_ts` header. Since all events in the pipeline produced using an older version of arroyo may not have the header yet, temporarily support a None value for `orig_message_ts`.

- Replaces Offset in consumer and processing strategy with Position, which contains both offset and timestamp information. `stage_offsets` is now `stage_positions` and `commit_offsets` is now `commit_positions`, and now includes the timestamp.

- Add orig_message_ts field to Commit and commit_codec. This field is included in the Kafka payload as a header.

- Add optional initializer function to parallel transform step. Supports passing a custom function to be run on multiprocessing pool initialization.