MXS-2002 Rename Worker::post() to Worker::execute()

The main point is that tasks/functions are executed, not that
they are posted.

server/core/resource.cc

@@ -1280,7 +1280,7 @@ HttpResponse resource_handle_request(const HttpRequest& request)
     mxs::Semaphore sem;
     ResourceTask task(request);
 
-    worker->post(&task, &sem, mxs::Worker::EXECUTE_AUTO);
+    worker->execute(&task, &sem, mxs::Worker::EXECUTE_AUTO);
     sem.wait();
 
     return task.result();