A MessageCombiner allows two messages in route to the same vertex to be aggregated into a single message.
Message combining can reduce the number of messages sent between vertices and thus, reduce network traffic.
Not all messages can be combined and thus, this is an optional feature of a VertexProgram.
Combine two messages and return a message containing the combination.
In many instances, it is possible to simply merge the data in the second message into the first message.
Such an optimization can limit the amount of object creation.