There are only two messages: prepare_message and outgoing_file_message.
Actually only the prepare_message is the message we send on wire.
Flatten the namespace.
This is a definition example for gossip_digest_ack with all its sub
classes.
It can be used by the code generator to create the serializer and
deserializer functions.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>