TAJO-759: Fix findbug errors added recently.
TAJO-711: Add boilerplate Avro appender and scanner.
TAJO-711: Add Avro storage type to tests. Add boilerplate code to AvroAppender and AvroScanner.
TAJO-711: Add package-info.java and start fleshing out AvroAppender.
TAJO-711: Flesh out AvroAppender and add AvroUtils.
TAJO-711: Fix compile-time errors.
TAJO-711: Add support for avro.schema.url. Add Avro schemas to some tests.
TAJO-711: Update tests.
TAJO-711: All tests pass.
TAJO-711: Implement flush() for AvroAppender. Add test for testing AVRO_SCHEMA_URL.