Terminate executors that aren't needed.

If we launch an executor and then kill the task immediately after, make
sure we also terminate the executor when there are no other tasks.

Review: https://reviews.apache.org/r/11125
Make sure you clean up the cgroups test.

Sometimes the cgroup test fails because the directory is already there.
This should ensure that it gets cleanup up in that case before we exit.

Review: https://reviews.apache.org/r/11126
Turn down noise from ZooKeeper.

Review: https://reviews.apache.org/r/11127
Terminate correct tasks when a slave disconnects.

Previously, when a slave disconnected all tasks for that framework would
be removed and it would result in a bad state for a given framework.  In
the case of Hadoop, it would result in a bunch of zombie tasks running
on the slaves which never terminate.

Review: https://reviews.apache.org/r/11128
More cgroup killTask() logging.

Review: https://reviews.apache.org/r/11129
Double JVM memory overhead.

Review: https://reviews.apache.org/r/11130
For cgroups killTasks(): kill, freeze, kill, thaw.

Review: https://reviews.apache.org/r/11131
Don't accept offers on the same host/port.

We'll try to find a port combination that hasn't already been used to
launch a TaskTracker.
Call `post-reviews' with --guess-fields argument.