kiji-schema: SCHEMA-682. Fix DebugResourceTracker UnsupportedOperationException on iterator.remove

issue: https://jira.kiji.org/browse/SCHEMA-682
review: https://review.kiji.org/r/1741
over-the-shoulder-reviewers: ['daisy']
commit: https://github.com/kijiproject/kiji-schema/commit/e69b3d423caaca04ef4c28ebf98b778b1075aa1f