kiji-express-music: EXP-298. Rename ColumnRequest[Input|Output] to Column[Input|Output]Spec

issue: https://jira.kiji.org/browse/EXP-298
review: https://review.kiji.org/r/1371
commit: https://github.com/kijiproject/kiji-express-music/commit/6726a3bb89e8b41e3a974faecba273febcdbebd8