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
it is byte slice with general usage, so call it context.

the context in AddNode is used to give some other info about the new node. we have to put PeerURLs into it, so etcdserver knows where to connect the new node. Besides that, it could contain name, clientURLs.