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/schemarouter/schemarouterinstance.hh

@@ -37,7 +37,7 @@ public:
     ~SchemaRouter();
     static SchemaRouter* create(SERVICE* pService, char** pzOptions);
     SchemaRouterSession* newSession(MXS_SESSION* pSession);
-    void diagnostics(DCB* pDcb);
+    json_t* diagnostics();
     uint64_t getCapabilities();
 
 private: