raft: add removed and rmNodes

The usage of rmNodes:
1. tell removed node about its removal explicitly using msgDenied
2. prevent removed node disrupt cluster progress by launching leader election

removed indicates whether the node has been removed. It is set when apply
removal of itself, or receive msgDenied.

#1016 
ShouldRemove: r.ShouldRemove()

r.ShouldRemove() bool {
  return r.removed[r.id]
}