log of the raft state machine shoud be store

[geffzhang]

https://ramcloud.atlassian.net/wiki/download/attachments/6586375/raft.pdf
Replicated state machines are typically implemented using a replicated log, as shown in Figure 1. Each server stores a log containing a series of commands, which its state machine executes in order. Each log contains the same commands in the same order, so each state machine
processes the same sequence of commands. Since the state machines are deterministic, each computes the same state and the same sequence of outputs