HTTP2: Fix handling of RST_STREAM with error = NO_ERROR after response EndStream (dotnet/corefx#39882)
* fix handling of RST_STREAM with error = NO_ERROR after response EndStream
* eat exception and ensure we don't cancel the response body
* add comment
* add CancelResponseBody to avoid code duplication, and add checks we are not holding the stream object lock when we don't expect to
* fix _expect100ContinueWaiter to use TaskCreationOptions.RunContinuationsAsynchronously
Commit migrated from https://github.com/dotnet/corefx/commit/
ca6c44b72b868c112387b0fb7def42d91f5577aa