* Establishes for the client link identified by 'req' a subscription
* to the 'event'.
*
+ * Establishing subscription MUST be called BEFORE replying to the request.
+ *
* @param req the request
* @param event the event to subscribe
*
* link identified by 'req'.
* Returns 0 in case of successful subscription or -1 in case of error.
*
+ * Revoking subscription MUST be called BEFORE replying to the request.
+ *
* @param req the request
* @param event the event to revoke
*
* 4. 'info' a string handling some info (can be NULL)
* 5. 'req' the req
*
+ * NOTE: For convenience, *json_object_put* is called on 'object' after the
+ * callback returns. So, it is wrong to call *json_object_put* in the callback.
+ *
* @param req The request
* @param api The api name of the method to call
* @param verb The verb name of the method to call