Cleanup snapshot mr jobs

- Set BytesWritable capacity
- Do not recreate PrioritizedFactBytes
- Rename PrioritizedFactBytes to PriorityTag
- Change outputKey to BytesWritable mutating it for each key
> This looks suspect, can these functions be allocated outside of the map functions. Similar to strings, we don't want to be allocating a new one per record.

I'm curious how the scala compiler will ETA `commit`. I'm not 100% it's going to be allocation free.