Handle partial failure from mget/mset

Hello @nicks, 

Please review the following commits I made in branch 'xiao-handle-partial-failure'.

74756569a40ee78142c8e8900f6aa3b1aea0be15 (2014-01-14 09:18:02 -0800)
Handle partial failure from mget/mset

64b174835a5976d957777799633b76221c256823 (2014-01-13 23:43:43 -0800)
Let CacheCluster use HashRing for consistent hashing

R=@nicks