Add consistent hash test.

Add consistent hash test.

P value is calculated as following:
when we add/remove M upstreams from the list, the probability of hash
miss should be close to the relation N / (N + M), where N is the size of
the previous upstreams list.

So far, the algorithm described in http://arxiv.org/abs/1406.2294 shows
p value more than 0.99.