Collect NIL nodes on binheap down percolation

[Ianleeclark]

This is essentially an optimization for the key expiration, as if we expire > 1 key at a time, we'll be left with > 1 nil block at a time. What I want to do is to treat a block of nil nodes as a single node and percolate the block down normally. I think this will reduce total time complexity from O(MlogN) to O(logN) where N is the number of total nodes in the binary heap and M is the number of nil nodes at the head of the list.

• Percolate chunks of nil nodes down as if they're a single node