This serves as the container about all of the state of a message exchange.
The objects presented to the binding and engine are instances of
MessageExchangeProxy. The MessageExchangeProxy forwards all get/set
requests to here. This class maintains a reference to each.
author: Sun Microsystems, Inc.
Create a new message exchange for the given proxy and exchange.
USed by the Observer mechanism to create the observed at that point in time.
Parameters: proxy - MessageExchangeProxy for pattern