Added support to assign all threads of a process to a cgroup.

Review: https://reviews.apache.org/r/19699
Added support to look up thread ids using /proc on Linux.

Review: https://reviews.apache.org/r/19361
Changed cgroup verification to accept an empty cgroup to permit specifying the
root cgroup.

Review: https://reviews.apache.org/r/19700
Added support to move the slave into separate cgroup for monitoring.

Review: https://reviews.apache.org/r/19248