Cleanup snapshot mr jobs

- Set BytesWritable capacity
- Do not recreate PrioritizedFactBytes
- Rename PrioritizedFactBytes to PriorityTag
- Change outputKey to BytesWritable mutating it for each key
i get the following compile error when i change this to map

```
[error] /Users/raronson/Documents/workspace/ivory/ivory-extract/src/test/scala/com/ambiata/ivory/extract/mr.scala:32: No implicit view available from => List[org.specs2.matcher.MatchResult[Any]] => org.scalacheck.Prop.
[error]   def e1 = prop((fs: List[Fact]) => {
[error]                ^
```
How can i fix this?