allow threads to append a series of graphs
allow threads to append a series of graphs

Previous incarnation basically allowed each thread to append only one
graph. This wastes effort. Use a bespoke mutex lock instead.

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