default to 25000 variants per region
default to 25000 variants per region

This provides the best performance on a 48-core machine, and for small graphs,
less than this size, we don't get much benefit out of multithreading. The
overheads of the reduce part of the process nullify the improvement.

ekg/vg,abeconnelly/vg,dnanexus-rnd/vg,ekg/vg,abeconnelly/vg,ekg/vg,dnanexus-rnd/vg