kiji-express: EXP-350. Support multiple HFile output taps in single flow

issue: https://jira.kiji.org/browse/EXP-350
review: https://review.kiji.org/r/1528
commit: https://github.com/kijiproject/kiji-express/commit/a6328d1e6fc1a1886c3aaf66a9500e0b4e96b903