main/raft: write bootstrap entry when start raft

goal:
1. write bootstrap entry into log
2. determine the data written into bootstrap entry

non-goal:
1. apply bootstrap entry
2. support recovery after full down
Why is this field called "Context"? How about just calling it "Cluster"?

If this doesn't sound right, maybe you could help me understand what it represents (e.g. what is the third argument to `EtcdServer.AddNode`?)