raft: add removed

The usage of removed:
1. tell removed node about its removal explicitly using msgDenied
2. prevent removed node disrupt cluster progress by launching leader election

It is set when apply node removal, or receive msgDenied.
raft: refine comment for doc and removed list tests
etcdserver: fix test due to rebase
Merge pull request #1151 from unihorn/138

raft: add removed