kiji-express: EXP-446. Add counters to the job history table for flow steps in KijiExpress.

- Adds job history to a new express_job_history table.
- Fetches and stores flow step metrics for each express job.

issue: https://jira.kiji.org/browse/EXP-446
review: https://review.kiji.org/r/1923
commit: https://github.com/kijiproject/kiji-express/commit/cc8a1d058b61dd1d403d60f0a735088ae2d99820