refactor(http): add http handler without creating http service

[neverchanje]

What problem do I solve?

To register an HTTP handler, we previously needed to define an http_service subclass before writing the exact handler.

class recent_start_time_http_service : public http_service
{
public:
    recent_start_time_http_service()
    {
        register_handler("",
                         std::bind(&recent_start_time_http_service::get_recent_start_time_handler,
                                   this,
                                   std::placeholders::_1,
                                   std::placeholders::_2),
                         "ip:port/recentStartTime");
    }

    std::string path() const override { return "recentStartTime"; }

    void get_recent_start_time_handler(const http_request &req, http_response &resp);
};

As we can see, http_service is too heavy to implement a lightweight handler.

How do I solve this problem

We can simplify this short type handler by using register_http_call, a new API of the library.

    register_http_call("recentStartTime")
        .with_callback([](const http_request &req, http_response &resp) {
            get_recent_start_time_handler(req, resp);
        })
        .with_help("Gets the server start time.");

Remember that we use register_http_service to register an http_sevice. I did to http_call likewise. register_http_call is designed to be a convenience utility that could boost our future development of new HTTP API.

Known issue

In this PR I removed version_http_service. This http call will be registered during a pegasus_service_app is created. So I will commit another PR for this.

[levy5307]

I think the comment doesn't make sense. Maybe you can remove it.

[neverchanje]

Sure. Any other problems?

[levy5307]

no