Remove the SourceProgress

This is a preliminary step versus https://bugs.launchpad.net/oq-engine/+bug/1248237
The goal is to simplify the relation between realizations and sources. Instead of a temporary table in the database a simple dictionary is enough to do the job. Here sources_per_rlz associates to a realization_id a list of parsed_source_ids. Point sources are special-cased. All tests pass: http://ci.openquake.org/job/zdevel_oq-engine/187