It is accessed by at least two threads:

* `[shutoff]` in `UnregisterHTTPHandler`
* `[http]` in `http_request_cb`
{
    switch (m) {
    case HTTPRequest::GET:
        return "GET";
        break;
    case HTTPRequest::POST:
        return "POST";
        break;
    case HTTPRequest::HEAD:
        return "HEAD";
        break;
    case HTTPRequest::PUT:
        return "PUT";
        break;
    default:
        return "unknown";
    }
}
static void http_request_cb(struct evhttp_request* req, void* arg)