Add Randomized AppendEntriesReply test

[allengeorge]

SImilar to the AppendEntries test. It would be useful to come up with a randomized AppendEntriesReply test that exercises:

1. Out of order replies.
2. A mix of nack and ack replies.
3. Varying numbers of added entries (to ensure that the nextIndex doesn't shrink).

...

[allengeorge]

The major thing missing from this implementation is a checking how onAppendEntriesReply can handle a mix of NACK and ACK replies. This is equivalent to putting the algorithm in either the PREFIX_SEARCH or APPLYING phase and then bombarding it with random messages. I'm not quite sure if there's a lot of value in that, although I'm open to revisiting this.