use abbreviated triples

[nttoldj](https://github.com/miku/nttoldj) 1.0.13 added
experimental abbreviated-namespace nttriples export.

For GND 07/2014 the abbreviated file is about 50% of the
size of the original, or about 7G. Expected LevelDB graph
will be around 15G.
import benchmark decorator
switch to closest-style dates
task for abbreviated freebase ntriples
create is enough, do not append