Merge github.com:grpc/grpc into bye-bye-completion-queue-pie

doc/interop-test-descriptions.md

include/grpc/support/useful.h

src/core/channel/context.h

src/core/profiling/basic_timers.c

src/core/profiling/stap_probes.d

src/core/profiling/stap_timers.c

src/core/profiling/timers.h

src/core/support/slice_buffer.c

src/core/surface/call.c

src/core/surface/call.h

src/core/transport/stream_op.c

src/core/transport/transport.h

src/csharp/.gitignore

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

src/csharp/Grpc.Core/AsyncClientStreamingCall.cs

src/csharp/Grpc.Core/AsyncDuplexStreamingCall.cs

src/csharp/Grpc.Core/AsyncServerStreamingCall.cs

src/csharp/Grpc.Core/Call.cs

src/csharp/Grpc.Core/Calls.cs

src/csharp/Grpc.Core/Channel.cs