Merge pull request #2860 from jtattermusch/compression_api

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

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

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

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

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

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

src/csharp/Grpc.Core/CallOptions.cs

src/csharp/Grpc.Core/CompressionLevel.cs

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

src/csharp/Grpc.Core/IAsyncStreamWriter.cs

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

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

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

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

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

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

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

src/csharp/Grpc.Core/ServerCallContext.cs

src/csharp/Grpc.Core/WriteOptions.cs

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