ISE when AppendEntriesReply received from a peer

[allengeorge]

Reported by @ZymoticB and noticed in testing on AWS.

Trace from #40

[New I/O worker #4] TRACE io.libraft.algorithm.RaftAlgorithm - agent2: RequestVote from agent1: term:2 lastLogIndex:15 lastLogTerm:1
[New I/O worker #4] INFO io.libraft.algorithm.RaftAlgorithm - agent2: changing role LEADER->FOLLOWER in term 2
[New I/O worker #4] INFO io.libraft.algorithm.RaftAlgorithm - agent2: leader changed from agent2 to null
[New I/O worker #4] TRACE io.libraft.algorithm.RaftAlgorithm - agent2: AppendEntriesReply from agent1: term:2 prevLogIndex:6 entryCount:9 applied:false
[New I/O worker #4] ERROR org.jboss.netty.channel.SimpleChannelUpstreamHandler - agent2: uncaught exception processing rpc:AppendEntriesReply{source=agent1, destination=agent2, term=2, prevLogIndex=6, entryCount=9, applied=false} from agent1
java.lang.IllegalStateException: role:FOLLOWER

This situation can be triggered by the following sequence of events:

Initial State:

• P1 = LEADER
• P2 = FOLLOWER
• Term = 1

1. P1 sends AppendEntries to P2 in term 1.
2. P2 has an election timeout and advances to term 2.
3. P2 sends a REQUEST_VOTE to P1.
4. P1 receives REQUEST_VOTE and transitions to term 2 and becomes FOLLOWER.
5. P2 receives AppendEntries sent in step 1.
6. P2 responds with AppendEntriesReply with term 2 and applied = false.
7. P1 receives AppendEntriesReply and crashes.