QUIC: simplified stream initialization.

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.