Remove length field from OBJECT_DATAGRAM

[afrind]

The length is inferred from the transport datagram. Add a new datagram type for conveying an Object Status. This saves 1-2 bytes per datagram.

Fixes: #552

[suhasHere]

I am not sure if there is a needed for explicit message for reporting status. Can't we just use the status in the original message.

[afrind]

@suhasHere The current DATAGRAM message has a status field that is present when length is 0. Since this PR removes the length field, the only thing I could see was either 1) include the status in every datagram, wasting space in the common case, or 2) define a special datagram message that carries only the status.

Do you have an alternative encoding to what is here that is unambiguous and compact?

[afrind]

Here's an alternate with a "single" message (single in scare quotes because there are technically 5 code points):

OBJECT_DATAGRAM Message {
  TypeAndStatus(i) 0x00..0x04
  Track Alias (i),
  Group ID (i),
  Object ID (i),
  Publisher Priority (8),
  [Payload(..)]
}

The first varint in the datagram is TypeAndStatus. Values 0-4 convey an Object. If TypeAndStatus is 0, the Status is Normal and a Payload exists to the remainder of the datagram. It is an error if the payload is empty. TypeAndStatus 1-4 indicate the given object status and have no payload. It is an error if the datagram has bytes beyond Publisher Priority. TypeAndStatus > 4 are undefined.

[suhasHere]

wonder if we can define the encoding in a way that we don't need 2 messages.