Cleanup snapshot mr jobs

- Set BytesWritable capacity
- Do not recreate PrioritizedFactBytes
- Rename PrioritizedFactBytes to PriorityTag
- Change outputKey to BytesWritable mutating it for each key
Just a question - should we be returning the results for this stuff (eg. do a map instead of foreach)? I know there is a bug in the current version of specs2 (but perhaps we can bump a minor version?)