clients: add util function.

추가된 유틸리티 함수:
* static std::string Info::str(const int &n);

Change-Id: I2ec60c9b8c0fdba25374f9d283c7058502859192
cpp clients: move socket-open location.

기존에는 소켓 open 시점이 FunapiClient 생성자였으나,
close 된 소켓 재활용이 가능하려면 Connect 메쏘드 안에서
소켓을 open 해야하므로 변경했음.

Change-Id: I05d03ba0227886540dd5c3d2c3c8dbde8413cd4f
cpp clients: minor naming change.

Change-Id: I4f1bcb641ae7addbd0a437ad4adb6e8d0e2bd905
cpp clients: update socket_fd access.

* Issue:1975

Change-Id: I1bab0216ef741bdc4a0a3e58627ec376be146e94
cpp clients: reset session_id on Session timeout.

Change-Id: I2591972a56fc8981484f93308c1a281e6c438ee4