Let CacheCluster keep track of partial failure count

Hello @nicks, @dpup, 

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

f10072f91566c4a21f62a5c86770a7eb0543abe0 (2014-01-27 15:47:10 -0800)
Let CacheCluster keep track of partial failure count

R=@nicks
R=@dpup