snap: error on empty snapshot
Merge pull request #1894 from xiang90/fix_snap

snap: error on empty snapshot
raft: increase term to 1 before append initial entries

Because the term of new raft is 0, it is weird to have term-1 committed
entries in the log.
Merge pull request #1888 from yichengq/258

raft: increase term to 1 before append initial entries
etcd: update conf when apply the confChange entry
raft: drop nodes in softState