Simplify MultiWriteCacheGroup to have only two instances, primary and secondary

Hello @nicks, @giannic, 

Please review the following commits I made in branch 'xiao-pair-cache'.

f0a44678ba3539aa0c45043a5fde95ea26657130 (2014-03-25 14:02:27 -0700)
Simplify MultiWriteCacheGroup to have only two instances, primary and secondary

R=@nicks
R=@giannic