fix(websocket): keep reconnect backoff when the upgrade fails

[prestonbrown]

TcpClient resets the reconnect delay and retry count in onconnect, as soon as TCP connects. For WebSocketClient that is before the upgrade request is even sent, so a server that accepts TCP but rejects the upgrade (404 on the path, Sec-WebSocket-Accept mismatch, a reverse proxy whose backend is down) puts the client in a reconnect loop at min_delay that never backs off and never reaches max_retry_cnt. WebSocketClient now saves cur_delay and cur_retry_cnt when TCP connects and restores them on close unless the connection reached WS_OPENED, so a working session still reconnects fast after it drops. unittest/websocket_reconnect_test.cpp points a client with exponential backoff at a server that answers 404 and checks the backoff state at the fourth close: master gives retries=1 cur_delay=0, this branch gives retries=3 cur_delay=40.

make unittest && LD_LIBRARY_PATH=lib bin/websocket_reconnect_test