EDS - Add CloseSend call to BackfilledStream

[friedemannf]

For consistency reasons, add a stream.CloseSend() call to the BackfilledStream's ReceiveFromStream.

According to this grpc-go commit, this doesn't seem to be actually needed, since the only two cases in which this is actually called are:

1. The stream has been read until Recv() returns a non-nil error
2. The context is cancelled, which is the same context that's passed to NewClientStream via getStreamWithRetry