SSL: save sessions for upstream peers using a callback function. In TLSv1.3, NewSessionTicket messages arrive after the handshake and can come at any time. Therefore we use a callback to save the session when we know about it. This approach works for < TLSv1.3 as well. The callback function is set once per location on merge phase. Since SSL_get_session() in BoringSSL returns an unresumable session for TLSv1.3, peer save_session() methods have been updated as well to use a session supplied within the callback. To preserve API, the session is cached in c->ssl->session. It is preferably accessed in save_session() methods by ngx_ssl_get_session() and ngx_ssl_get0_session() wrappers.