Fix the dependency on the blocksize

See https://bugs.launchpad.net/oq-engine/+bug/1225287

The idea is to have a separate seed for each source (i.e. use blocksize=1): however this would generate a large amount of tasks with the current distribution mechanism. This pull request change the distribution mechanism,
so that the arguments generated by .task_arg_gen() are grouped in chunks and only a task per chunk is spawned.
The mechanism makes sure that the number of spawned tasks is below a maximum which is ten times the concurrent_tasks parameter (a good heuristic). See the docstring of parallelize for details.
All the tests pass: http://ci.openquake.org/job/zdevel_oq-engine/136/console