Don't prematurely kill TaskTrackers.

We wait for 300,000ms (5 minutes) of inactivity before we kill a running
TaskTracker in case there are still reducers who need the task results.

You may override the timeout by setting the number of milliseconds in
mapred-site.xml with the property 'mapred.tasktracker.idle.timeout'.

In our testing, we found that on very long jobs the TaskTrackers would
be killed before reducers got the results of the map tasks.  We use a 30
minute idle timeout to ensure the TaskTrackers aren't terminated
prematurely.
Hadoop tutorial version bump (CDH4.2.0 -> 4.2.1).
Shuffle offers before sending to frameworks.

This helps ensure fair allocation of resources.
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.
Log executor output to syslog.