Cannot compute response time for every request

For some requests, kr_layer_t::req->rplan.initial == NULL applies, meaning we don't have access to the members of initial's kr_query, particularly creation_time_mono, where the timestamp should be stored.

We've observed this behavior when sending repeated requests for the same domain. On the first request, initial is not NULL, but for all subsequent requests with the same domain name, initial becomes NULL.

For some requests, kr_now() - kr_layer_t::req->rplan.initial->creation_time_mono == 0 applies, meaning we can't compute correct response time.

We've observed this behavior when sending repeated requests for the same domain. On the first request, kr_now() - kr_layer_t::req->rplan.initial->creation_time_mono != 0, but for all subsequent requests with the same domain name kr_now() - kr_layer_t::req->rplan.initial->creation_time_mono == 0.

(I've updated description based on the further investigation - #928 (comment 313969))