[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.
Even if we drop the aggregator, you still don't think it'd be a good idea to keep either the sampleId or the countAtPosition fields?