Lower HSDir query backoff delay

At the moment this value (REND_HID_SERV_DIR_REQUERY_PERIOD) equals 15m. It's pretty long. E.g. if one tries to reach an onion service which descriptor has not yet been published (due to chaotic uptime [ricochet], etc) they're unable to reach this service for 15m. See some discussion about it in legacy/trac#20082 (moved). Should it be 15m or something less? If something else then why?