QLowEnergyService::writeCharacteristic - implement timeout logic for Darwin OS

When we write a characteristic with write mode 'WriteWithResponse' we rely on our delegate and a special callback from CoreBluetooth. Unfortunately, there is no notion of a timeout in CoreBluetooth, so it can happen we never have any response but never report any error either. Like it is done with different discovery methods in QLowEnergyController, we have to introduce 'external' timeout handling (here 'external' means Qt does this, not CoreBluetooth).