multicast and setting header

[Tyler Durvik]

I have a group of processors that I am sending messages to via
multicast.  Each processor will add a header to the message based on
the what the processor needs to do.  The problem is that not all
headers are in the message after the multicast has completed.   I have
the multicast set to parallelprocessing=true, so I am not sure if this
makes the exchange read-only maybe.