Message: write/read the payload last when marshalling

When (un)marshalling a message, we process the fields in this order:

1. Sender and receiver addresses
2. Flags
3. Payload
4. Headers

I'd like to change this to move processing of the payload to the end because

• If payload refers to a different buffer (JGRP-1773), then we don't need to read the bytes, but currently have to skip N bytes to read the headers. If the payload was at the end, this wouldn't be needed
• Once we use NIO.2 and gathering writes, we'd pass a couple of buffers to the socket write and therefore it's simpler if the payload is written last. Ditto for scattering reads.