SOAP MEPs

I believe I had an action from last Wednesday to write up what the two
new SOAP MEPs might look like.  Here is a quick sketch of what I
originally had in mind.  Having thought it over a bit more, I'm not sure
we need all of this, but that's for a separate message.

According to some set of rules, for example those published previously,
a sender of a request message (i.e., a client) decides whether it is
engaging in

    * A SOAP in-only MEP
    * A SOAP in-optional-out MEP (where the out may be a reply or a
      fault, depending on the details)
    * A SOAP in-out MEP

This would be indicated by the SOAP MEP property (as currently defined
in the SOAP 1.2 adjuncts).  This property is currently omitted because
the value is always clear from context: A synchronous HTTP in-out always
uses the in-out MEP.   In order to take full advantage of WSA, it will
sometimes need to be present, whether as a SOAP header, a
transport-level header, or whatever (on could argue it's always present
in the status quo, through out-of-band agreement).

In the HTTP world

    * One-way is just the first half of the current request-response. 
      We don't care what happens after the request half, because the
      client is going to ignore it.  Alternatively, we could require the
      server to send back a special marker, perhaps 202 or 204.
    * In-only consists of the first half of hate current
      request-response.  The server MUST either send back a reply
      message, a fault message or a special marker, again perhaps 202 or
      204.  The special marker tells the client that the actual response
      was redirected elsewhere, as per the MAPs the client included
      (apologies if I've once again switched "reply" and "response" here).
    * The in-out MEP is the same as it ever was.  If there is no
      explicit indication of the SOAP MEP, this is the default.

In the strict one-way world, only the first option (in-only) is allowed.