prop224: HSdir index desynch between client and service

David found his client unable to connect to his service. Apparently, they compute different hsdir indices, since it was 12:20UTC (non-overlap period) and the live consensus had valid-after at 11:00UTC (overlap period). Apparently something got confused.

Theory: We use time(NULL) as the time in node_set_hsdir_index() whereas we use the live consensus valid-after in rotate_all_descriptors(). This can cause desynch within the same tor instance. We should probably use the live consensus valid-after in all cases to have a common point of reference, and avoid problems with clock skews.