[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.
I was going to remove the aggregator; it's not as useful in practice as I originally thought it would be, and it puts implementation constraints on the rest of the pileup code.