A server stream can be reset by the client while server writes are still queued. After the stream is reset, the netty connection will forget the stream object. The `NettyServerHandler` must deal with that situation. `sendResponseHandlers` already had some code to do that. This change standardizes that code and adds it to `sendGrpcFrame`. This fixes a potential bug where a `SendGrpcFrameCommand` with `endOfStream=true` would raise an `AssertionError` if written to a reset stream. (This bug is not currently reachable because `endOfStream=false` for all server `SendGrpcFrameCommand` objects.)