Fast export of the GMFs

See https://bugs.launchpad.net/oq-engine/+bug/1321544. The improvement here is essential to make it possible to export the GMFs for our demos from the platform in an reasonable amount of time.
In particular the demos/risk/ScenarioRisk export time goes down to 1m16s (was 4 minutes and before 46 minutes).

The tests are green: https://ci.openquake.org/job/zdevel_oq-engine/846