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 
i test all message types to ensure it could react correctly for any input. it is not based on implementation.
msgDeny should never happen. i will remove it from this test.