update AppendEntriesFailureResponse#expectedNextIndex to speed up log index match

[JoeCqupt]

speed up log index match

[JoeCqupt]

@metanet PTAL

[metanet]

thanks for your pull request @JoeCqupt.

sorry for my late reply. i am usually quite busy with work on weekdays and mostly available on weekends.

no idea what the change is doing. can you provide a description of the change, how does it speed up follower catch up and why it is safe?

[JoeCqupt]

as the author mentioned in his paper that the protocol can be optimized to reduce the number of rejected
AppendEntries RPCs

dissertation#basicraft/consensus.tex#L403

I referred to some optimizations of existing systems：

• sofa-jraft
  sofa-jraft handle AppendEntriesRequest rejection by return follower's lastLogIndex

• ratis
  ratis handle InconsistentAppendEntries basically by return follower's lastCommittedIndex