revamp of c# channel options

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

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

...arp/Grpc.Core.Tests/Internal/ChannelArgsSafeHandleTest.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/ChannelOptions.cs

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

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

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

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

src/csharp/ext/grpc_csharp_ext.c