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
To put it another way: if `Bootstrap` is a type of `ConfChange`, it should share some of the fields that the other `ConfChange` types use. Otherwise it's just being shoe-horned in there with a generic field of arbitrary contents.