Cleanup snapshot mr jobs

- Set BytesWritable capacity
- Do not recreate PrioritizedFactBytes
- Rename PrioritizedFactBytes to PriorityTag
- Change outputKey to BytesWritable mutating it for each key
@etorreborre Posted the fix the other day, there is an implicit that needs to be added until we go to the next specs version. @etorreborre Do you have the List -> Result implicit handy still?