kiji-express: EXP-395. SpecificRecords persisted to disk in between FlowSteps will come back as GenericRecords.

issue: https://jira.kiji.org/browse/EXP-395
review: https://review.kiji.org/r/1549
commit: https://github.com/kijiproject/kiji-express/commit/9247b8563fe07b9f684477aa6730f7960011fefe