Make unknown container not a Failure for Isolator::cleanup.

Review: https://reviews.apache.org/r/22049
Add a cgroup perf_event isolator.

This isolator only creates the perf_event cgroup and places the executor
into the cgroup; no performance statistics are gathered/exported.

Review: https://reviews.apache.org/r/20963
Introduce a PerfStatistics protobuf.

Review: https://reviews.apache.org/r/21442
Support for running "perf stat".

Perf can be run against either a set of pids or a set of perf_event cgroups.

Review: https://reviews.apache.org/r/21443
Add sampling support to the perf_event isolator.

Review: https://reviews.apache.org/r/21451