a copycat implementation to learn raft
todo
- 异步响应 append log
- 使用内存状态替代 storage 的读写
- 简化 storage 对象
- 干掉 storage 中的 commitIndex
- lock
- commit command
references
https://github.com/codymalick/simple-raft https://github.com/radondb/xenon/blob/master/src/raft/raft.go https://github.com/streed/simpleRaft https://github.com/Qihoo360/floyd/tree/master/floyd/src https://github.com/hashicorp/raft/blob/c95aa91e604eaafe4174d7ae0f404afa247f60c4/raft.go https://zhuanlan.zhihu.com/p/27415397