Cleanly exit a forked executor if the slave terminates during launch.

Review: https://reviews.apache.org/r/22481
Fixed a flaky test: SlaveTest.TerminatingSlaveDoesNotReregister.

Review: https://reviews.apache.org/r/22472
Fixed ZooKeeperProcess and ProcessWatcher to avoid deadlock during ZooKeeperProcess termination.

Review: https://reviews.apache.org/r/22521
Fixed SlaveRecoveryTest.RestartBeforeContainerizerLaunch test.

Review: https://reviews.apache.org/r/22441
Removed TODO that was done.

Review: https://reviews.apache.org/r/22577
Added grace period to health check and moved health check into TaskInfo.

Review: https://reviews.apache.org/r/22519
Fixed typo in health check grace periods field.

Review: https://reviews.apache.org/r/22589
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
Verbose Maven configure error.

Review: https://reviews.apache.org/r/21950
Adding missing files to gitignore template.

Review: https://reviews.apache.org/r/21718
Give the command executor some cpu and memory.

Review: https://reviews.apache.org/r/22251
Wait on Containerizer::update to complete before sending status update.

Review: https://reviews.apache.org/r/22253
Fix some clang compiler issues in perf code.
Minor style fix.
Enabling ZooKeeper tests that were previously disabled.

Review: https://reviews.apache.org/r/22591
MESOS-1312: Added orphan_tasks and unregistered_frameworks in
master's state.json.

Added orphan_tasks and unregistered_frameworks in state.json to display
tasks that are currently running on the slaves, but do not belong to
any already (re-)registered frameworks.

Review: https://reviews.apache.org/r/22223