Addition of developer and user modes to debugcli

In order to be able to protect the interface so that mistyped argument
do not endanger MaxScale yet still allow the developer access a router
option has been added that gives the debugcli two modes.

server/include/session.h

@@ -87,6 +87,7 @@ typedef struct session {
 
 SESSION	*session_alloc(struct service *, struct dcb *);
 bool    session_free(SESSION *);
+int	session_isvalid(SESSION *);
 void	printAllSessions();
 void	printSession(SESSION *);
 void	dprintAllSessions(struct dcb *);