changed the way ports are added to the server

src/csharp/Grpc.Core.Tests/ClientServerTest.cs

src/csharp/Grpc.Core.Tests/ServerTest.cs

src/csharp/Grpc.Core.Tests/TimeoutsTest.cs

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.Core/Server.cs

src/csharp/Grpc.Core/ServerPort.cs

src/csharp/Grpc.Examples.MathServer/MathServer.cs

src/csharp/Grpc.Examples.Tests/MathClientServerTests.cs

src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs

...csharp/Grpc.IntegrationTesting/InteropClientServerTest.cs

src/csharp/Grpc.IntegrationTesting/InteropServer.cs

src/csharp/Grpc.IntegrationTesting/SslCredentialsTest.cs