Use pipes in CombinedReadable

Hello @evansolomon,

Please review the following commits I made in branch 'es-combined-with-pipe'.

428a00f316bb767c489a810a5ba5c727cdc608d1 (2014-03-14 11:44:07 -0700)
Use pipes in CombinedReadable
This makes it simpler to manage backpressure.

R=@evansolomon