QUIC: simplified stream initialization. After creation, a client stream is added to qc->streams.uninitialized queue. After initialization it's removed from the queue. If a stream is never initialized, it is freed in ngx_quic_close_streams(). Stream initializer is now set as read event handler in stream connection. Previously qc->streams.uninitialized was used only for delayed stream initialization. The change makes it possible not to handle separately the case of a new stream in stream-related frame handlers. It makes these handlers simpler since new streams and existing streams are now handled by the same code.