Merge pull request #2648 from jtattermusch/csharp_logging_api

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/AsyncCallBase.cs

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

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

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

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

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

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

src/csharp/Grpc.Core/Server.cs

src/csharp/Grpc.Core/Utils/BenchmarkUtil.cs

src/csharp/Grpc.IntegrationTesting/SslCredentialsTest.cs