pairing can fail

[Florian Zinggeler]

Pairing a device can fail if for some reason, the websocket is blocked on one device, but not the other.
In that case we can easily get into an inconsistent state.

Solving this problem completely is impossible, but we can do better than now.

All it would take is another round of acknowledgements, as this way the second one can't make it if the websocket of one device doesn't work