avro source/sink should not use file header

[xiangjinwu]

When using confluent schema registry, there is a 5-byte header during serialization:
https://docs.confluent.io/platform/current/schema-registry/fundamentals/serdes-develop/index.html#messages-wire-format

When not using confluent schema registry, we have several options:

• No header. Like our current implementation for json or protobuf. (from_avro_datum/to_avro_datum)
• Avro single-object encoding (ref) header. (GenericSingleObjectReader/GenericSingleObjectWriter)
• Avro object container file (ref) header. (Reader/Writer)

The current avro source (only append-only allows no schema registry) and sink (unreleased yet) uses the file header, which is huge and intended to be shared by multiple records within the same file. As messages in a queue, we shall switch to one of the earlier options.