MR-279: Add a getCounter(Enum) method to the Counters interface

I'm fixing a few TODOs I came across in TaskAttemptImpl.java related to the fact that the MRv2 Counters interface don't expose a getCounter(Enum) method for accessing a Counter using the enum's class as the group name and the enum's value as the name of the counter.

Will add the patch momentarily.