QUIC: asynchronous shutdown.

QUIC: asynchronous shutdown.

Previously, when cleaning up a QUIC stream in shutdown mode,
ngx_quic_shutdown_quic() was called, which could close the QUIC connection
right away.  This could be a problem if the connection was referenced up the
stack.  For example, this could happen in ngx_quic_init_streams(),
ngx_quic_close_streams(), ngx_quic_create_client_stream() etc.

With a typical HTTP/3 client the issue is unlikely because of HTTP/3 uni
streams which need a posted event to close.  In this case QUIC connection
cannot be closed right away.

Now QUIC connection read event is posted and it will shut down the connection
asynchronously.