[RFC] Integrate dyanmic loading to let users easy to use.

[jovany-wang]

This RFC aims to discuss how we support the very easy to use RAFT service.

I propose this simple usage:

// my_counter_service.h

class MyCounterService {
public:
    virtual int get() const = 0;
    virtual void incr(int delta) = 0;
};

// my_counter_service_impl.h

class MyCounterServiceImpl : public MyCounterService {
public:
    int get() const { return val_; }
    void incr(int delta) { val_ += delta; }

private:
    int val_ = 0;
};

INIT_RAFT_SERVICE(MyCounterService, MyCounterService::incr, MyCounterService::get);

/// server.h

#include "my_counter_service_impl.h"

int main() {
    raftcpp::Loop();
}

/// client.h

#include "my_counter_service.h"
int main() {
}

[jovany-wang]

@xzhangxian1008 @zhangdexin @qicosmos @BryainZhang

[jovany-wang]

Note that this is very important to support our slogan:
A Raft implementation to help build your Raft service in 1 minute.