MXS-1220: Add JSON return value to diagnostics entry points

The modules that implement a diagnostics entry point now return a JSON
type object. This removes the need to format data inside the modules.

The module implementations of these are not yet complete which means that
MaxScale will fail to compile.

include/maxscale/filter.h

@@ -23,6 +23,7 @@
 #include <maxscale/dcb.h>
 #include <maxscale/routing.h>
 #include <maxscale/session.h>
+#include <maxscale/jansson.h>
 
 MXS_BEGIN_DECLS
 
@@ -174,9 +175,12 @@ typedef struct mxs_filter_object
      *
      * @param instance Filter instance
      * @param fsession Filter session, NULL if general information about the filter is queried
-     * @param dcb      DCB where the diagnostic information should be written
+     *
+     * @return JSON formatted information about the filter
+     *
+     * @see jansson.h
      */
-    void     (*diagnostics)(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, DCB *dcb);
+    json_t* (*diagnostics)(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
 
     /**
      * @brief Called to obtain the capabilities of the filter