Represent the object that nodes gossip around the network.
### builder(): [MessageBuilder](#messagebuilder)
Creates a new `MessageBuilder` to construct an instance of a `Message`.
### networkId(): long
Returns the network id of a `Message`.
### id(): [MessageId](#messageid)
Computes the identifier of the message.
### nonce(): long
Returns the nonce of a `Message`.
### parents(): [MessageId[]](#messageid)
Returns the parents of a `Message`.
### payload(): Optional<[MessagePayload](#messagepayload)>
Returns the optional payload of a `Message`.