job tracker should keep a log of task completion and failure

The JobTracker should track a list of task completion events in JobInProgress.

So JobClientProtocol & InterTrackerProtocol should get a new method:

TaskCompletionEvent[] getTaskCompletionEvents(String jobid, int fromEventId) throws IOException;

TaskCompletionEvent should have:
int getEventId();
String getTaskTrackerHttp();
String getTaskId();
static public enum Status

;
Status getTaskStatus();
}

The events will be stored in a List<TaskCompletionEvent> and the eventId is the position in the list.

These event logs will allow JobClient to display task output to the user as well as provide the start of the fix for HADOOP-248.