Javadoc
Sets this member to slave mode, to sync up its Raft log to match the specified address.
In slave mode:
the member contacts the specified address, which is expected to be a leader
once successfully connected, the member becomes a follower
the member ignores all RequestVote messages
if the connection fails, the member continues to retry connecting to the specified address
After this call, the commitIndex and savedIndex will be 0.
If masterAddress is -1, this member leaves slave mode.
Any waiters for committed commands will get a SLAVE_MODE exception when slave is entered.