SCHEMA-523. Avro schemas in TableLayoutDesc should be specified as schema ID or schema JSON descriptor.

- Introduces AvroSchema descriptor (UID or a JSON representation).
- Revamp the TableLayoutBuilder to use AvroSchema.
- TableLayoutBuilder now normalizes to schema UIDs and automatically sets a layout ID.

issue: https://jira.kiji.org/browse/SCHEMA-523
review: https://review.kiji.org/r/1038