Eflist

I have tested the code on some datasets. It turns out that when the graph is small, the spill size is more likely to be 0. However, the spill size will be more than 640KB for larger datasets, such as twitter2010 and clueweb12. The bvline graph and head_tail_1000 graph are not very good for testing eflist code since the offsets files are very large for these synthetic graphs. It took a lot of time to scan the graph to generate the eflists and it also took a lot of time to generate the index structures.
We shouldn't have two different arrays, because both offsetbuf and efbuf won't be used simultaneously.