QUIC: added stateless reset support. The new "quic_stateless_reset_token_key" directive is added. It sets the endpoint key used to generate stateless reset tokens and enables feature. If the endpoint receives short-header packet that can't be matched to existing connection, a stateless reset packet is generated with a proper token. If a valid stateless reset token is found in the incoming packet, the connection is closed. Example configuration: http { quic_stateless_reset_token_key "foo"; ... }