kiji-schema: SCHEMA-644 Added tweet avrò record, and new layout that uses RKF2 w hashed encoding.

Testing done: mvm clean install.  Some tests on new layout, some on legacy.
    issue: https://jira.kiji.org/browse/SCHEMA-644

    review: https://review.kiji.org/r/1777/
commit: https://github.com/kijiproject/kiji-schema/commit/1c787cddfe7709eb01d665fea5c28d5f676452c5