kiji-express: 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/1365
commit: https://github.com/kijiproject/kiji-express/commit/8fbe3774794c1ea5344dec87b9c9de11efe20807