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.
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.