[CDH4] Updates to IoMgr from adding cached read support in C5.

To minimize the merge cost in the IoMgr code going forward, this is a
cherry-pick of the CDH5 changes with the minimal amount of code removed
to run on CDH4.

Change-Id: I66cb8d0db75e7cad678d3bb4fcf11b338c1446ae
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1455
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins