Status of a message transfer on a {Link} Common base class for {Tracker} and {Delivery}.
@return [Connection] The parent connection.
@return [String] Unique ID for the transfer in the context of the {link}
@return [Link] The parent link.
@deprecated internal use only
@return [Session] The parent session.
@return [Integer] Remote state of the transfer, one of the values in {State}
@return [Transport] The parent connection‘s transport.
[Validate]