Wait for the WebSocket to be open (i.s.o. the TCP connection)

Now the SmartLinkType<> waits till the WebSocket included the upgrade exchange before reporting that it is open.

In the past the Open would already return if the TCP connection was established, however, the socket is than not yet fully open, it first has to request the upgrade. If, before the socket was fully opened, the first messages where to be exchanged, they got lost.