MXS-2149 Add REST-API watchdog

This will simply cause a task to be posted to each worker.
If the workers are running normally, the task will reach the
workers and the associated semaphore posted, and the REST-API
call will return. If any worker is not running normally, the
task will not be processed and the REST-API call will hang.

server/core/resource.cc

@@ -900,6 +900,7 @@ HttpResponse cb_modulecmd(const HttpRequest& request)
 
 HttpResponse cb_send_ok(const HttpRequest& request)
 {
+    mxs_rworker_watchdog();
     return HttpResponse(MHD_HTTP_OK);
 }