Parallelize the queries of kind "insert into gmf agg" by using the standard mechanism

This is an improvement over https://bugs.launchpad.net/oq-engine/+bug/1178054; instead of parallelizing in a ad hoc way, we use the same approach used in gmf_to_hazard_curve_task, i.e. we parallelize by site. I have also added an UNIQUE constraint to the gmf_agg table to prevent incorrect aggregations.