MXS-1220: Treat service listeners as service sub-resources

The listeners under the /services/:service/listeners collection are now
fully JSON API compliant resources.

The listeners could also be exposed as a /listeners collection to easily
group all listener type resources in one place. This approach does has
some semantical and practical problems, namely the fact that each listener
has a many-to-one relationship with its service and listeners by
themselves can't exist alone.

include/maxscale/service.h

@@ -304,6 +304,16 @@ json_t* service_to_json(const SERVICE* service, const char* host);
  */
 json_t* service_list_to_json(const char* host);
 
+/**
+ * @brief Convert service listeners to JSON
+ *
+ * @param service Service whose listeners are converted
+ * @param host    Hostname of this server
+ *
+ * @return Array of JSON format listeners
+ */
+json_t* service_listeners_to_json(const SERVICE* service, const char* host);
+
 /**
  * @brief Get links to services that relate to a server
  *