Remove unused FileSystem import
Change ValueState to handle any ThriftLike value
Extract common setup for priority/namespaced mappers
Changed okCounter var to val
Added type alias MutableFact
Extract PriorityTagDerializer for reducing prioritized thrift values
Pass State function into PriorityTagDeserializer constructor to ensure correct handling
Merge pull request #197 from ambiata/topic/priority-mr-utils

Extract a few Priority-based helpers around MR jobs
using a constrained Name for namespaces names