完善rpc功能

[RFoe]

https://github.com/RFoe/coasync/blob/a466688370ce4bc5feefc8a55d960de447dfbfde/include/coasync/net/rpc/rpc_client.hpp#L10 https://github.com/RFoe/coasync/blob/a466688370ce4bc5feefc8a55d960de447dfbfde/include/coasync/net/rpc/rpc_server.hpp#L21

rpc功能还是过于简单

• 设置请求超时，或许是这样：

  co_await client.call<int>("add", 888, 999) | timeout(3s);

• 设置连接/通信异常触发动作，或许是这样：

  
  net::rpc::rpc_server server {std::move(acceptor), [](std::error_code& ec) {...;} };
  co_await server.start() | on_error{[](std::error_code& ec) {...;}};

co_await client.call("add", 888, 999) | on_error([](std::error_code& ec) {...;});

+ 优雅的抑制响应/断开连接
```cpp
// common definition
enum option_set {
    suppress_response_requested = 0,
    shut_down_requested,
}
// server side
rpc_server.bind("generic_control", [] (net::rpc::control_token token, int option) -> void
{
    if(option == shut_down_requested) token.request_close();
    else if (option == shut_down_requested) token.request_noresponse();
    return;
});
// client side
co_await rpc_client.call_nonblock<void>("generic_control", option_set::suppress_response_requested);
co_await rpc_client.call_nonblock<void>("generic_control", option_set::shut_down_requested);

• 注册中心/分布式服务发现和负载均衡（还不太了解，先不做）