SSL: support for TLSv1.3 early data with BoringSSL. Early data AKA 0-RTT mode is enabled as long as "ssl_early_data on" is specified in the configuration (default is off). The $ssl_early_data variable evaluates to "1" if the SSL handshake isn't yet completed, and can be used to set the Early-Data header as per draft-ietf-httpbis-replay-04.