add send() function to client

include/protocol.hpp

@@ -46,6 +46,14 @@ class generic_message_base
     spdlog::debug("create_message is not implemented yet");
   }
 
+  virtual void handle_sent(const boost::system::error_code& er) {
+    if (!er) {
+      spdlog::debug("Message was sent");
+    } else {
+      spdlog::error("Error occured on client::send");
+    }
+  }
+
  protected:
   bool is_handler = true;
 };