MXS-1220: Implement JSON diagnostics for most routers

All routers except the binlogrouter now fully implement the JSON
diagnostic entry point. The binlogrouter needs to be handled in a separate
commit as it produces a large amount of diagnostic output.

server/modules/routing/hintrouter/hintrouter.hh

@@ -22,7 +22,7 @@ class HintRouter : public maxscale::Router<HintRouter, HintRouterSession>
 public:
     static HintRouter* create(SERVICE* pService, char** pzOptions);
     HintRouterSession* newSession(MXS_SESSION *pSession);
-    void diagnostics(DCB* pOut);
+    json_t* diagnostics();
     uint64_t getCapabilities() const
     {
         return RCAP_TYPE_NONE;