MAX_PAYLOAD_SIZE in connection.rs and MAX_PACKET_SIZE in parameter.rs are not consistent. We should consider put them together.

[fanlong]

[sparkmiw]

The MAX_PACKET_SIZE is currently a rough and conservative estimation for the message size. It is now only used when constructing message for returning blocks. Making MAX_PACKET_SIZE and MAX_PAYLOAD_SIZE consistent requires some design pattern improvement in the network layer, which we can first discuss about it.