ETP DevKit (branch etp1.2) failed to dispose EtpSession object and websocket connection

1.Deadlock happened when EtpSessionNativeBase.Dispose() method was called, making the DevKit failed to dispose the session object.
2.After fixing the deadlock issue, another issue was spotted: when closing the websocket connection, the session was already disposed, so the "acquiring send lock" process throwed the exception and broke in EtpSession.CloseWebSocketAsync().
