Simplify Server Initialization about raft

[xiang90]

Only allow the first log entry to be a self-join.
The first log entry should be a join, but not necessarily a self-join i think.

[benbjohnson]

@xiangli-cmu The first needs to be a self join since it'll need to be replicated to peers when other nodes join. If node A starts and adds B, then A fails and B becomes leader, but then B never knows about A since there wasn't a log entry.

[xiang90]

A is the first node. It sends join-A, which is a self-join. Then becomes leader.
A 's log entries will be join-A and nop.

B tries to join the cluster of A. It sends join-B. Then A will commit join-B.
A 's log entries will be join-A , nop and join-B.

Then A send B appendEntries request. B will accept them.
B 's log entries also will be join-A , nop and join-B.

Correct me, if I do not understand it right.

[benbjohnson]

@xiangli-cmu yes, that's correct.