Display both task ID and task attempt ID in UI, and rename taskId to taskAttemptId

Previously only the task attempt ID was shown in the UI; this was confusing because the job can be shown as complete while there are tasks still running.  Showing the task ID in addition to the attempt ID makes it clear which tasks are redundant.

This commit also renames taskId to taskAttemptId in TaskInfo and in the local/cluster schedulers.  This identifier was used to uniquely identify attempts, not tasks, so the current naming was confusing.  The new naming is also more consistent with map reduce.