make vcf-compare-gt use less memory

the stop coordinate of the region being tracked by the vcf
GenotypeComparator was never getting reset, leading to excessive memory
usage.

specifically, for each chromosomes after the first one, all variants on
that sequence were being loaded and hashed (which kept the large
Vcf::Entry objects in memory) instead of clearing out the hash table
whenever we cross a gap between two ordered variants.