trade.Order now has TraderId and TradeId fields

Previously this was represented by the field Guid. But the guid
field has been pushed into the prioq package so the data structures
sent over the network as sensible and straight forward.

As a nice side-effect the horrible mkGuid and getTraderId/getTradeId are
now private in the prioq package.
Package guidstore -> guid

Moved guid helper functions MkGuid(...), GetTraderId(...) and
GetTradeId(...) out of the prioq/ package and into guid/

This was done because the netwk.Listener needs to be able to generate
guids from incoming orders in order to store them in the guidstore. So
we converted the guidstore package into a generic guid package to allow
everyone to work with guids easily.
MATCHER_ACK and SHUTDOWN messages supported

Including tests in the netwk package.

Also merged OrderKind and ResponseKind into one type MsgKind. It is very
likely that this preceeds the merging of Order and Response into a
single Message struct.

Various cleanups also sneaked in.