Avoid redundant writes across clusters that share same some cache servers

Hello @nicks, @sboora, @giannic, 

Please review the following commits I made in branch 'xiao-avoid-redudant-double-write'.

4e64d03553ad7f9197ddfa1896d8350bc7f4a3fb (2014-03-19 21:39:30 -0700)
Avoid redundant writes across clusters that share same some cache servers

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