raft: prev should be set only when we sucessfully send out rd to the channel
raft: record previous state is enough
raft: set ready.st to empty state, if current state is equal to previous state
wal: do not save empty state
raft: add a newReady helper function
raft: remove extra code
wal: test save empty state
raft: use IsEmptyState
Merge pull request #1026 from coreos/fix_node

Fix node