Let CacheCluster use HashRing for consistent hashing

Hello @nicks, 

Please review the following commits I made in branch 'xiao-use-hashring'.

456edd2165368bed1176aaab9f2aa4340f904ca9 (2014-01-10 13:46:24 -0800)
Let CacheCluster use HashRing for consistent hashing

R=@nicks