SSL: variables support in ssl_certificate and ssl_certificate_key. To evaluate variables, a request is created in the certificate callback, and then freed. To do this without side effects on the stub_status counters and connection state, an additional function was introduced, ngx_http_alloc_request(). Only works with OpenSSL 1.0.2+, since there is no SSL_CTX_set_cert_cb() in older versions.