Merge pull request #6699 from jtattermusch/csharp_streaming_api_improvements

src/csharp/Grpc.Core.Tests/Internal/AsyncCallServerTest.cs

src/csharp/Grpc.Core.Tests/Internal/AsyncCallTest.cs

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

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/AsyncCompletion.cs

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

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