kiji-rest: REST-79. Don't close the underlying Kiji instance.

The underlying Kiji instance was being closed by the table/instance endpoint
which was causing the later read/write endpoints to bomb because the shared Kiji
instance was being released.

Reviewer: shashir, lsheng
commit: https://github.com/kijiproject/kiji-rest/commit/1ead136c30474f8d5b8a90ea155869c9f1ffd05a