Fixed the State output operators.

Review: https://reviews.apache.org/r/13954
Fixed a CHECK failure in the master when successive exited events
occur for a disconnected slave.

Review: https://reviews.apache.org/r/13956
Fixed a bug in reconciliation that failed to account for unknown
executors.

Review: https://reviews.apache.org/r/13921
Added a cgroups::memory namespace.

Review: https://reviews.apache.org/r/13903
Fixed CgroupsIsolator to listen for OOMs of recovered executors.

Review: https://reviews.apache.org/r/13904
Exposing cpu.stat statistics in the monitoring endpoint.

From: Christina Delimitrou <christina.delimitrou@gmail.com>
Review: https://reviews.apache.org/r/13868