Read missed keys from secondary server

Hello @nicks, @giannic, @sboora, 

Please review the following commits I made in branch 'xiao-read-from-secondary-server'.

fa731f83059724f4839e4dbd3ec83a7b8a088ec5 (2014-03-20 14:25:10 -0700)
Read missed keys from secondary server
In a following up PR, I will make MultiWriteCacheGroup to have
only two servers, one primary and one seconday. It doesn't seem
to very useful to have multiple write-only servers.

R=@nicks
R=@giannic
R=@sboora