HTTP/3: postponed session creation to init() callback. Now the session object is assigned to c->data while ngx_http_connection_t object is referenced by its http_connection field, similar to ngx_http_v2_connection_t and ngx_http_request_t. The change allows to eliminate v3_session field from ngx_http_connection_t. The field was under NGX_HTTP_V3 macro, which was a source of binary compatibility problems when nginx/module is build with/without HTTP/3 support. Postponing is essential since c->data should retain the reference to ngx_http_connection_t object throughout QUIC handshake, because SSL callbacks ngx_http_ssl_servername() and ngx_http_ssl_alpn_select() rely on this.