Replicate method calls the AppendEntries method on servers without skipping itself #52

Hi, @wenweihu86 , I think there's an issue in Code Snippet 1 because configuration.getServersList also includes the node itself. If peerMap.get(server.getServerId()) refers to the node itself, the return value will be null, and calling appendEntries(null) will trigger a null pointer exception(see Code Snippet 2 peer.getNextIndex()).

Suggested fix:

for (RaftProto.Server server : configuration.getServersList()) { 
+    if (server.getServerId() == localServer.getServerId()) {
+        continue;
+    }
    final Peer peer = peerMap.get(server.getServerId());
    executorService.submit(new Runnable() {
        @Override
        public void run() {
            appendEntries(peer);
        }
    });
}

I'm looking forward to your confirmation, and would be happy to help fix the issue if needed.