fix

[dragonylffly]

[dragonylffly]

@PFZheng

[dragonylffly]

@chenzhangyi As the codes as well as comments in Lines 86-88 ballot_box.cpp, the elements before last_committed_index are removed. So it just needs to find the largest last_committed_index, use reverse search to avoid calling bl.granted() and updating last_committed_index multiple times