In the event based calculator split by ruptures, not by SES

See https://bugs.launchpad.net/oq-engine/+bug/1262473 for an explanation. With this patch, the Hannover RE computation can be done in the cluster in 3 minutes or so, and many of our QA tests become faster. The GMF-generation part of the event based calculator becomes much simpler by removing various special cases.

All tests pass: http://ci.openquake.org/job/zdevel_oq-engine/238/