kiji-express: EXP-388. Use buffered writer for direct kiji output

issue: https://jira.kiji.org/browse/EXP-388
review: https://review.kiji.org/r/1545
commit: https://github.com/kijiproject/kiji-express/commit/c9de5be9c7961eab0eee30be08baf3bd3bcb64bc