private void linkIn(Entry entry) {
        Entry head = slots[entry.slot];
        if (head == null) {
            entry.next = entry;
            entry.prev = entry;
            slots[entry.slot] = entry;
        } else {
            entry.next = head;
            entry.prev = head.prev;
            head.prev.next = entry;
            head.prev = entry;
        }
        ++counts[entry.slot];
    }
@author <a href="mailto:havardpe@yahoo-inc.com">Haavard Pettersen</a>
@author Simon Thoresen Hult
jdisc_core/src/main/java/com/yahoo/jdisc/core/ScheduledQueue.java