change kmer indexing
change kmer indexing

Cache all our kmers in memory, sorted. Then write to rocksdb in sorted order.

Fingers crossed-- this will improve write performance on large datasets.

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