redo security (aka. access control)

Break up monolithic SecurityPlugin API into three parts.

1. Authentication.
2. Authorization.
3. Access Control decision.  Left to ChannelProviders

testApp/remote/testCodec.cpp

@@ -396,13 +396,7 @@ public:
 
     void aliveNotification() {}
 
-    void authNZMessage(epics::pvData::PVField::shared_pointer const & data) {}
-
-    virtual std::tr1::shared_ptr<SecuritySession> getSecuritySession() const
-    {
-        return std::tr1::shared_ptr<SecuritySession>();
-    }
-
+    void authNZMessage(epics::pvData::PVStructure::shared_pointer const & data) {}
 
 
     bool isClosed() {