Add cache performance stats to the Redis connection

Hello @ArtemTitoulenko, @don-neufeld, 

Please review the following commits I made in branch 'xiao-add-metrics'.

c0d0fe836b44275866d34f6dbc27b85a416b9b95 (2013-07-31 22:41:55 -0700)
Add cache performance stats to the Redis connection


R=@ArtemTitoulenko
R=@don-neufeld