Perform the most minimal parse possible to calculate the length of the message payload.
This is only required for subclasses of ChildMessage as root level messages will have their length passed
into the constructor.
Implementations should adhere to the following contract: If parseLazy = true the 'length'
field must be set before returning. If parseLazy = false the length field must be set either
within the parseLite() method OR the parse() method. The overriding requirement is that length
must be set to non UNKNOWN_MESSAGE value by the time the constructor exits.