Replication tests and diff_batch_size

We are likely not exercising the code paths when a fullsync syncs more keys than diff_batch_size, given that it defaults to 100 keys *per vnode*, and I believe we test with the default ring size of 64. So unless we write 6,400 keys and then fullsync, we aren't going to hit that code.

A bug was just found: basho/riak_repl#199 which *should* have been caught by our riak tests, but was not precisely because of this issue.

I think simply lowering the diff_batch_size for the tests, to something like 10, and making sure we write more than 640 keys in one pass before we do a fullsync should do the trick.

cc @buddhisthead @lordnull @metadave