private HashedWheelTimeout pollTimeout() {
            HashedWheelTimeout head = this.head;
            if (head == null) {
                return null;
            }
            HashedWheelTimeout next = head.next;
            if (next == null) {
                tail = this.head = null;
            } else {
                this.head = next;
                next.prev = null;
            }

            // null out prev and next to allow for GC.
            head.next = null;
            head.prev = null;
            head.bucket = null;
            return head;
        }
Clear this bucket and return all not expired / cancelled {@link Timeout}s.
jraft-core/src/main/java/com/alipay/sofa/jraft/util/timer/HashedWheelTimer.java