Merge github.com:google/grpc into async-api-new

src/csharp/GrpcCore/Call.cs

src/csharp/GrpcCore/GrpcCore.csproj

src/csharp/GrpcCore/IMarshaller.cs

src/csharp/GrpcCore/Internal/AsyncCall.cs

src/csharp/GrpcCore/Internal/ServerSafeHandle.cs

src/csharp/GrpcCore/Internal/ServerWritingObserver.cs

src/csharp/GrpcCore/Internal/StreamingInputObserver.cs

src/csharp/GrpcCore/Method.cs

src/csharp/GrpcCore/Server.cs

src/csharp/GrpcCore/ServerCallHandler.cs

src/csharp/GrpcCore/ServerCalls.cs

src/csharp/GrpcCoreTests/ClientServerTest.cs

src/csharp/GrpcCoreTests/ServerTest.cs