Use ThriftSerialiser for everything - no exceptions
Merge pull request #199 from ambiata/topic/thrift-clear-spec

Add ThriftDerserializationBehaviourSpec to demonstrate clear behaviour
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