Add MultiplexingCache that delegates to another cache

Hello @x-ma, 

Please review the following commits I made in branch 'dpup-multiplex'.

1e11c3e60f8bf96f0c5d16aae1ffa0c49575ee76 (2014-02-04 15:10:14 -0800)
Add MultiplexingCache that delegates to another cache

R=@x-ma