Respond now resending unacked messages

All response messages of kind BUY, SELL or CANCEL must be acknowledged
by the client or they will be resent. They are currently being resent
every second.
Merged Order and Response into Message

package trade/ -> msg/

This major refactoring was motivated by the observation that the server
was sending and receiving the same messages.

If we take a BUY order as an example. The order needs to be acknowledged
with a MATCHER_ACK message which contains all the information in the BUY
order. Here we can see that the server is sending messages which look
like orders.

If the BUY order produces a FULL match we can see the opposite.
The FULL match becomes a response that is sent back to the client. But
the client now has to acknowledge the FULL match message. So a
CLIENT_ACK comes back into the server corresponding to the response we
sent previously. Here we can see that the server is receiving messages
which look like responses.

So now all messages look the same. They are distinguished by their Kind
value. The increasingly misnamed trade/ package finally has a sensible
name.
Split MsgKind into MsgKind and MsgRoute

This is done in order to separate the notion that a message is an ORDER
or an ACK etc. with whether it is a BUY, SELL or a CANCEL etc.

For instance a message can be a BUY ORDER, this order will need to be
acked so we can now produce a BUY ACK message. Previously if you were a
BUY then you weren't an ACK and vice versa.
Using real matcher in netwk tests

The netwk package's tests now use the real matcher instead of the mock
we were using previously. This makes the netwk package's unit tests into
a full system integration test. This isn't so bad, but it could confuse
people who expect simple unit tests.

We now have a testing approach which builds up from the simple
components, such as prioq and msg, reusing dependent components for the
tests at each stage. I am comfortable with this approach at this time,
but may in the future move integration style tests into a separate
testing area (but then again I might not).

Also snuck in a COMMAND MsgRoute and moved SHUTDOWN into MsgKind as
shutdown message now has Route=COMMAND and Kind=SHUTDOWN. This was done
so that we had a coherent SERVER_ACK for the shutdown message, which
previously had the kind NO_KIND.