Merge pull request #2872 from jtattermusch/fix_preconditions

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

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

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

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

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

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

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

src/csharp/Grpc.Core/CallInvocationDetails.cs

src/csharp/Grpc.Core/CallOptions.cs

src/csharp/Grpc.Core/Calls.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/ChannelOptions.cs

src/csharp/Grpc.Core/ClientBase.cs

src/csharp/Grpc.Core/ContextPropagationToken.cs

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

src/csharp/Grpc.Core/GrpcEnvironment.cs

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

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

src/csharp/Grpc.Core/KeyCertificatePair.cs

src/csharp/Grpc.Core/Logging/ConsoleLogger.cs