A protocol message that contains a repeated series of block headers, sent in response to the "getheaders" command.
This is useful when you want to traverse the chain but know you don't care about the block contents, for example,
because you have a freshly created wallet with no keys.
Perform the most minimal parse possible to calculate the length of the message.
This is only required for subclasses of ChildClass 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.