Optimize commit_index calculation

[akiradeveloper]

Calculating commit_index is done by leader and later propagated to followers. Simply saying, the algorithm is list the replicated index of all the servers and find the center value.

Let n be the number of servers in the cluster. Naively, the computation is the order of O(n logn) and it is trivial when n is small. It can be O(n) optimally but it will be practically slower because the coefficients are larger as the code is complicated.

In the worst case, this calculation is done n time to increment one commit_index. So in total we consume O(n^2 logn) calculation per one log entry. If n is big (say 1000) this will be non-trivial. (I don't know someone to manage n=1000 Raft cluster)

If we have bitmask where bit 1 is "replication index is updated since last commit_index update", n is replaced with popcnt which is interpreted as CPU instruction because if number of 1s in the bitmask is smaller than the majority number, the computation can be skipped.

Disadvantage: N will be limited to at most 64 if we are adhere to call popcnt only once. This is practically okay I believe.