Simplified the task distribution in the risk calculators

See https://bugs.launchpad.net/oq-engine/+bug/1376157
A test to check task-number independence has been added too (https://bugs.launchpad.net/oq-engine/+bug/1309378)
Notice that the companion PRs https://github.com/gem/oq-commonlib/pull/43 and https://github.com/gem/oq-risklib/pull/77 are needed.

The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/732