[BDG-FORMATS-26] Removing bulky fields from the pileup record.

This pull request removes "bulky" fields from the pileup record. The read group fields in the pileup record lead to a large memory/GC overhead and are ultimately unnecessary for most pileup related tasks. This also removes the aggregation specific fields from the pileup record.
@fnothaft in retrospect (and this is my conclusion, after looking at the downstream PileupAggregator code in adam) aren't we going to need the ``countAtPosition`` field?  In particular, I think that the code in PileupAggregator that aggregates ``mapQuality`` and ``sangerQuality`` needs the corresponding (sub)counts for each of the pileups it's aggregating...  If you say 'yes' to this question, I'll open a PR to add this field back in.