Server demos should show recommended server shutdown

[bernardnormier]

The recommended shutdown procedure for a server is to shut it down before destroying it. In some demos, the server is correctly shut down, for example:

ice-demos/cpp11/Ice/hello/Server.cpp

Line 37 in 7d29cb7

communicator->shutdown();

In other demos, the server is just destroyed. While it doesn't matter for those demos, it's not the recommended style:

ice-demos/csharp/Ice/hello/Server.cs

Line 24 in 7d29cb7

Console.CancelKeyPress += (sender, eventArgs) => communicator.destroy();

We should also consider not using communicator.waitForShutdown() in demos with no shutdown operation. All we need is a "WaitForCtrlC()".