another 5% improvement in indexing
another 5% improvement in indexing

Setting options.min_write_buffer_number_to_merge = threads seems to do it. As
usual, this needs to be verified on a heavyweight system, not just my lappy.

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