Fix TaskTracker pending tasks calculation.

Review: https://reviews.apache.org/r/11116
Ignore unhealthy TaskTrackers.

Review: https://reviews.apache.org/r/11117
Be more intelligent about slot allocation.

When we get a resource offer, we'll try to be clever about how we
allocate slots to the offers.  Ideally we want to use as much of the
resources available when we have a lot of pending tasks, and we also
don't want to use more resources than we actually need.

Review: https://reviews.apache.org/r/11118
Don't prematurely kill TaskTrackers.

We assign the JobID to the internal Mesos tracker (for the TaskTracker)
rather than the TaskID.

In the case where a TaskTracker has only map tasks assigned to it and
they have all completed, we mustn't terminate the tracker until the
entire job has finished, including the map tasks.

Review: https://reviews.apache.org/r/11119
Hadoop tutorial version bump (CDH4.2.0 -> 4.2.1).

Review: https://reviews.apache.org/r/11120
Shuffle offers before sending to frameworks.

This helps ensure fair allocation of resources.

Review: https://reviews.apache.org/r/11121
Slave feature: maximum system load.

When the load exceeds a specified value, don't accept tasks.  Some nodes
may become unstable under excessive load (i.e., heavy disk I/O), and
this helps prevent the assigning of further tasks to busy slaves.

Review: https://reviews.apache.org/r/11122
Log executor output to syslog.

Review: https://reviews.apache.org/r/11123
Kill tasks that never properly launch.

After trying to launch a task tracker, we'll wait up to 5 minutes before
giving up and killing the task.

Review: https://reviews.apache.org/r/11124
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
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.

Review: https://reviews.apache.org/r/11161/
Call `post-reviews' with --guess-fields argument.

But only do so with the initial creation of the review.

Review: https://reviews.apache.org/r/11159
Ignore errors from post-review.

When updating existing reviews, post-review returns errors erroneously.

Review: https://reviews.apache.org/r/11160
Write Mesos JobTracker stats to state file.

Review: https://reviews.apache.org/r/11165/
Fix post-reviews.py url.