ISSUE: Inconsistency in number of properties

There are currently four properties: InboundMessage, OutboundMessage,
ImmediateSender and ImmediateDestination.  The sender and receivers each
have a private copy of ImmediateSender and ImmediateDestination, but
OutboundMessage is only visible to senders and InboundMessage only to
receivers.

Since each node has its own copy of the properties, it would be more
consistent to have a single Message property.  The distinction of
inbound and outbound proved awkward in trying to write a binding [1] and
in describing possible differences between sender and receiver views of
the MEP [2].

Proposed change:

    * Delete the row in the property table for OutboundMessage
    * Rename InboundMessage to Message in the first row.
    * In the Property Description for Message, change

This property is populated if and only if the message is successfully
received.

    to

This property is populated at a receiver if and only if the message is
successfully received.

(By the way, we don't seem to say whether the ImmediateSender and
ImmediateDestination properties are only populated on success).

[1] http://lists.w3.org/Archives/Public/xml-dist-app/2006Aug/0035.html
[2] (not archived yet, probably 52)