Invalid reference counting

I found couple more places where .references == 0 means something else than DEAD, DON'T USE.

1. lib/dns/cache.c, the extra condition is cache->live_tasks > 0
2. lib/dns/zone.c, the extra condition is DNS_ZONE_FLAG(zone, DNS_ZONEFLG_SHUTDOWN) && isc_refcount_current(&zone->irefs) == 0)
3. lib/dns/resolver.c, the extra condition is fctx->pending == 0 && fctx->nqueries == 0 && ISC_LIST_EMPTY(fctx->validators))
4. lib/isc/task.c, the extra condition is EMPTY(task->events) && !TASK_SHUTTINGDOWN(task)
5. lib/ns/client.c, the extra condition is client->nsends == 0 && client->nrecvs == 0 and client->mortal && TCP_CLIENT(client) && client->newstate != NS_CLIENTSTATE_FREED && (client->sctx->options & NS_SERVER_CLIENTTEST) == 0

I am leaving lib/dns/rbtdb.c out of the list on purpose, but git grep "isc_refcount_current([^)]*) == 0" reveals all the places...