code: private synchronized void trimHead() {
      if(offsets.size()<=1) {
        return;
      }
      FileOffset head = offsets.first();
      FileOffset head2 = offsets.higher(head);
      if( head.isNextOffset(head2) ) {
        offsets.pollFirst();
        trimHead();
      }
      return;
    }
docs: e.g.:  1,2,3,4,10,11,12,15  =>  4,10,11,12,15