remove loop-locking from graph append
remove loop-locking from graph append

The construction of a subgraph is completed without blocking the further
progress of other threads through the while loop. The append of a completed
subgraph to the resulting graph is now done using the same pattern.

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