Started using immutable collections and other code cleanup

src/csharp/Grpc.Core/Calls.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/ChannelArgs.cs

src/csharp/Grpc.Core/Credentials.cs

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

src/csharp/Grpc.Core/Internal/ServerCallHandler.cs

src/csharp/Grpc.Core/Marshaller.cs

src/csharp/Grpc.Core/RpcException.cs

src/csharp/Grpc.Core/ServerCalls.cs

src/csharp/Grpc.Core/ServerCredentials.cs

src/csharp/Grpc.Core/ServerServiceDefinition.cs

src/csharp/Grpc.Core/StatusCode.cs

src/csharp/Grpc.Core/packages.config

...rp/Grpc.IntegrationTesting/Grpc.IntegrationTesting.csproj

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

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

src/csharp/Grpc.IntegrationTesting/TestCredentials.cs

src/csharp/Grpc.IntegrationTesting/packages.config