[cdc_rsync] Add support for ServerSocket on Windows (#48)

Makes ServerSocket multi-platform, mainly by working around some small
API differences. The code is largely the same, there should be no
differences on Linux.

Also moves WSAStartup() and WSACleanup() up to the Socket level as
static methods because it's used by both ClientSocket and ServerSocket,
and because it doesn't make sense to do that in the socket class as
that would prevent one from using several sockets.

cdc_rsync_server/server_socket.h

@@ -50,11 +50,7 @@ class ServerSocket : public Socket {
                        size_t* bytes_received) override;
 
  private:
-  // Listening socket file descriptor (where new connections are accepted).
-  int listen_sockfd_;
-
-  // Connection socket file descriptor (where data is sent to/received from).
-  int conn_sockfd_;
+  std::unique_ptr<struct ServerSocketInfo> socket_info_;
 };
 
 }  // namespace cdc_ft